P21? A day in the life of an Aztec midwife?

Lord Sun dawns on the day called 7-Monkey, his fingers slowly spreading a rosy sheen that mixes softly with smoke rising from Tenochtitlan’s many hearth fires. The midwife, Xoquauhtli, has a difficult choice to make. A momentous shift from rainy season to dry season is underway. All summer, the gods have kept the people fed with corn, but the fertile summer months are disappearing. This day occurs during the festival that marks the shift between the summer season, when the gods feed the people, and the winter season, when the people feed the gods in return. Xoquauhtli owes a debt to her patron, Teteoinnan, the female warrior goddess at the center of this festival. Teteoinnan wages war both on women’s battlefields of birth and in men’s battles with Tenochtilan’s enemies. She must be kept happy or she will bring bad luck. The midwife should participate in the festival today, but one of her patients could go into labor any minute. Xoquauhtli decides to check on her patient first. The expecting mother hasn’t worked too hard, chewed gum, or lifted heavy things. Her family is taking good care of her. Surely Xoquauhtli can take a little time to honor her goddess. She leaves her apprentice in charge and heads to the center of the city. Along the way, she sees women sweeping the roads and hanging gourds in preparation for the festival. Finally, she reaches the Great Pyramid. On top are two temples: the north, where rituals honor the rain god in the summer, and the south one is where rituals honor the war god in the winter. On the equinox, the sun rises between the two sides. The ceremony begins with a mock battle between the midwives and the other physicians. Xoquauhtli’s team battles heartily, throwing nochtles, marigolds, and balls made of reed and moss. They joke, call their rivals names, and laugh. But then, a girl comes running with a message for Xoquauhtli. Her patient is in labor! She hurries back to the house. All the old women from the extended family have already gathered for the birth— their experience is very valuable if anything goes wrong. She readies herself with a prayer praising her most important tools, her fingers. Then she doses the patient with cihuapatli to help expel the baby, massages her in the sweathouse, and rubs her stomach with tobacco. Offering Teteoinnan a short prayer, she urges her patient to act like a warrior. A strong baby girl slips into her waiting hands and the old women shout triumphant cries. Xoquauhtli takes a few drops of water from a jade bowl, breaths on them, and places them on the baby’s tiny tongue. She calls her a precious greenstone, a little warrior, and tells her how the Lord and Lady of the Ninth Sky breathed life into her, sending her to this place of burden and torment. She then turns to the new mother, praising her, telling her she acted like an eagle warrior, a jaguar warrior. By the time they finish, it’s late, and the flames of the fire have died down. Xoquauhtli piles the remaining hot coals in the center of the hearth, stoking them to keep them going. She lays the baby in a woven basket, head facing the warming fire. This will warm her tonalli, an important “soul” center in the body central to health and well-being. It’s almost midnight— if Xoquauhtli hurries, she can get back to the temple for the culmination of the festival. She makes her way to the city center, where a priest carries a woman on his back to the top of the pyramid. To begin the new season and feed the gods, she will be beheaded, symbolizing how corn is cut in the fields. Afterward, she will be reborn as Lady Teteoinnan, and preside over the induction of new warriors.

P22??A different way to visualize rhythm

We usually think of rhythm as an element of music, but it's actually found everywhere in the world around us, from the ocean tides to our own heartbeats, rhythm is essentially an event repeating regularly over time. Even the ticking of a clock itself is a sort of rhythm. But for musical rhythm, a steady string of repeating single beats is not enough. For that, we need at least one opposing beat with a different sound, which can be the unstressed off beat or the accented back beat. There are several ways to make these beats distinct, whether by using high and low drums, or long and short beats. Which ends up being heard as the main beat is not a precise rule, but like the famous Rubin's vase, can be reversed depending on cultural perception. In standard notation, rhythm is indicated on a musical bar line, but there are other ways. Remember that ticking clock? Just as its round face can trace the linear passage of time, the flow of rhythm can be traced in a circle. The continuity of a wheel can be a more intuitive way to visualize rhythm than a linear score that requires moving back and forth along the page. We can mark the beats at different positions around the circle using blue dots for main beats, orange ones for off beats, and white dots for secondary beats. Here is a basic two beat rhythm with a main beat and an opposing off beat. Or a three beat rhythm with a main beat, an off beat, and a secondary beat. And the spaces between each beat can be divided into further sub-beats using multiples of either two or three. Layering multiple patterns using concentric wheels lets us create more complex rhythms. For example, we can combine a basic two beat rhythm with off beats to get a four beat system. This is the recognizable backbone of many genres popular around the world, from rock, country, and jazz, to reggae and cumbia. Or we can combine a two beat rhythm with a three beat one. Eliminating the extra main beat and rotating the inner wheel leaves us with a rhythm whose underlying feel is three-four. This is the basis of the music of Whirling Dervishes, as well as a broad range of Latin American rhythms, such as Joropo, and even Bach's famous Chaconne. Now if we remember Rubin's vase and hear the off beats as the main beats, this will give us a six-eight feel, as found in genres such as Chacarera, and Quechua, Persian music and more. In an eight beat system, we have three layered circles, each rhythm played by a different instrument. We can then add an outermost layer consisting of an additive rhythmic component, reinforcing the main beat and increasing accuracy. Now let's remove everything except for this combined rhythm and the basic two beat on top. This rhythmic configuration is found as the Cuban cinquillo, in the Puerto Rican bomba, and in Northern Romanian music. And rotating the outer circle 90 degrees counterclockwise gives us a pattern often found in Middle Eastern music, as well as Brazilian choro, and Argentinian tango. In all of these examples, the underlying rhythm reinforces the basic one-two, but in different ways depending on arrangement and cultural context. So it turns out that the wheel method is more than just a nifty way of visualizing complex rhythms. By freeing us from the tyranny of the bar line, we can visualize rhythm in terms of time, and a simple turn of the wheel can take us on a musical journey around the world.

P23? A guide to the energy of the Earth

Energy is all around us, a physical quantity that follows precise natural laws. Our universe has a finite amount of it; it's neither created nor destroyed but can take different forms, such as kinetic or potential energy, with different properties and formulas to remember. For instance, an LED desk lamp's 6 Watt bulb transfers 6 Joules of light energy per second. But let's jump back up into space to look at our planet, its systems, and their energy flow. Earth's physical systems include the atmosphere, hydrosphere, lithosphere, and biosphere. Energy moves in and out of these systems, and during any energy transfer between them, some is lost to the surroundings, as heat, light, sound, vibration, or movement. Our planet's energy comes from internal and external sources. Geothermal energy from radioactive isotopes and rotational energy from the spinning of the Earth are internal sources of energy, while the Sun is the major external source, driving certain systems, like our weather and climate. Sunlight warms the surface and atmosphere in varying amounts, and this causes convection, producing winds and influencing ocean currents. Infrared radiation, radiating out from the warmed surface of the Earth, gets trapped by greenhouse gases and further affects the energy flow. The Sun is also the major source of energy for organisms. Plants, algae, and cyanobacteria use sunlight to produce organic matter from carbon dioxide and water, powering the biosphere's food chains. We release this food energy using chemical reactions, like combustion and respiration. At each level in a food chain, some energy is stored in newly made chemical structures, but most is lost to the surroundings, as heat, like your body heat, released by your digestion of food. Now, as plants are eaten by primary consumers, only about 10% of their total energy is passed on to the next level. Since energy can only flow in one direction in a food chain, from producers on to consumers and decomposers, an organism that eats lower on the food chain, is more efficient than one higher up. So eating producers is the most efficient level at which an animal can get its energy, but without continual input of energy to those producers, mostly from sunlight, life on Earth as we know it would cease to exist. We humans, of course, spend our energy doing a lot of things besides eating. We travel, we build, we power all sorts of technology. To do all this, we use sources like fossil fuels: coal, oil, and natural gas, which contain energy that plants captured from sunlight long ago and stored in the form of carbon. When we burn fossil fuels in power plants, we release this stored energy to generate electricity. To generate electricity, heat from burning fossil fuels is used to power turbines that rotate magnets, which, in turn, create magnetic field changes relative to a coil of wire, causing electrons to be induced to flow in the wire. Modern civilization depends on our ability to keep powering that flow of electrons. Fortunately, we aren't limited to burning non-renewable fossil fuels to generate electricity. Electrons can also be induced to flow by direct interaction with light particles, which is how a solar cell operates. Other renewable energy sources, such as wind, water, geothermal, and biofuels can also be used to generate electricity. Global demand for energy is increasing, but the planet has limited energy resources to access through a complex energy infrastructure. As populations rise, alongside rates of industrialization and development, our energy decisions grow more and more important. Access to energy impacts health, education, political power, and socioeconomic status. If we improve our energy efficiency, we can use our natural resources more responsibly and improve quality of life for everyone.

P24??A poetic experiment Walt Whitman, interpreted by three animators?

As an experiment, we gave recordings of the same Walt Whitman poem to 3 different animators. Each interpreted the text with no knowledge of what the others were creating Here's the result. "A Noiseless Patient Spider" by Walt Whitman Interpretation #1 by Jeremiah Dickey, Medium: Paint on Glass "A noiseless patient spider, I mark'd where on a little promontory it stood isolated, Mark'd how to explore the vacant vast surrounding, It launch'd forth filament, filament, filament, out of itself, Ever unreeling them, ever tirelessly speeding them. And you O my soul where you stand, Surrounded, detached, in measureless oceans of space, Ceaselessly musing, venturing, throwing, seeking the spheres to connect them, Till the bridge you will need be form'd, till the ductile anchor hold, Till the gossamer thread you fling catch somewhere, O my soul." Interpretation #2 by Biljana Labovic, Medium: Video "A noiseless patient spider, I mark’d where on a little promontory it stood isolated, Mark’d how to explore the vacant vast surrounding, It launch’d forth filament, filament, filament, out of itself, Ever unreeling them, ever tirelessly speeding them. And you O my soul where you stand, Surrounded, detached, in measureless oceans of space, Ceaselessly musing, venturing, throwing, seeking the spheres to connect them, Till the bridge you will need be form’d, till the ductile anchor hold, Till the gossamer thread you fling catch somewhere, O my soul." Interpretation #3 by Lisa LaBracio, Medium: Scratchboard "A noiseless patient spider, I mark’d where on a little promontory it stood isolated, Mark’d how to explore the vacant vast surrounding, It launch’d forth filament, filament, filament, out of itself, Ever unreeling them, ever tirelessly speeding them. And you O my soul where you stand, Surrounded, detached, in measureless oceans of space, Ceaselessly musing, venturing, throwing, seeking the spheres to connect them, Till the bridge you will need be form’d, till the ductile anchor hold, Till the gossamer thread you fling catch somewhere, O my soul."

P25??A simple way to tell insects apart?

A whip-like straw. Powerful, crushing blades. A pointed, piercing tube. There are nearly a million known insect species in the world, but most have one of just five common types of mouthparts. And that’s extremely useful to scientists because when they encounter an unfamiliar insect in the wild, they can learn a lot about it just by examining how it eats. Scientific classification, or taxonomy, is used to organize all living things into seven levels: kingdom, phylum, class, order, family, genus, and species. The features of an insect’s mouthparts can help identify which order it belongs to, while also providing clues about how it evolved and what it feeds on. The chewing mouthpart is the most common. It’s also the most primitive— all other mouthparts are thought to have started out looking like this one before evolving into something different. It features a pair of jaws called mandibles with toothed inner edges that cut up and crush solid foods, like leaves or other insects. You can find this mouthpart on ants from the Hymenoptera order, grasshoppers and crickets of the Orthoptera order, dragonflies of the Odonata order, and beetles of the Coleoptera order. The piercing-sucking mouthpart consists of a long, tube-like structure called a beak. This beak can pierce plant or animal tissue to suck up liquids like sap or blood. It can also secrete saliva with digestive enzymes that liquefy food for easier sucking. Insects in the Hemiptera order have piercing-sucking mouthparts and include bed bugs, cicadas, aphids, and leafhoppers. The siphoning mouthpart, a friendlier version of the piercing and sucking beak, also consists of a long, tube-like structure called a proboscis that works like a straw to suck up nectar from flowers. Insects of the Lepidoptera order— butterflies and moths— keep their proboscises rolled up tightly beneath their heads when they’re not feeding and unfurl them when they come across some sweet nectar. With the sponging mouthpart, there’s yet another tube, this time ending in two spongy lobes that contain many finer tubes called pseudotracheae. The pseudotracheae secrete enzyme-filled saliva and soak up fluids and dissolved foods by capillary action. House flies, fruit flies, and the other non-biting members of the Diptera order are the only insects that use this technique. But, there’s a catch. Biting flies within Diptera, like mosquitoes, horse flies, and deer flies, have a piercing-sucking mouthpart instead of the sponging mouthpart. And finally, the chewing-lapping mouthpart is a combination of mandibles and a proboscis with a tongue-like structure at its tip for lapping up nectar. On this type of mouthpart, the mandibles themselves are not actually used for eating. For bees and wasps, members of the Hymenoptera order, they serve instead as tools for pollen-collecting and wax-molding. Of course, in nature, there are always exceptions to the rules. The juvenile stages of some insects, for example, have completely different kinds of mouths than their adult versions, like caterpillars, which use chewing mouthparts to devour leaves before metamorphosing into butterflies and moths with siphoning mouthparts. Still, mouthpart identification can, for the most part, help scientists—and you —categorize insects. So why not break out a magnifying lens and learn a little more about who’s nibbling your vegetable garden, biting your arm, or just flying by your ear.

P26??Accents by Denice Frohman?

I'm Denice Frohman, and this is "Accents." my mom holds her accent like a shotgun, with two good hands. her tongue, all brass knuckle slipping in between her lips her hips, are all laughter and wind clap. she speaks a sanchocho of spanish and english, pushing up and against one another, in rapid fire there is no telling my mama to be "quiet," my mama don't know "quiet." her voice is one size better fit all and you best not tell her to hush, she waited too many years for her voice to arrive to be told it needed house keeping. English sits in her mouth remixed so "strawberry" becomes "eh-strawbeddy" and "cookie" becomes "eh-cookie" and kitchen, key chain, and chicken all sound the same. my mama doesn't say "yes" she says, "ah ha" and suddenly the sky in her mouth becomes a Hector Lavoe song. her tongue can't lay itself down flat enough for the English language, it got too much hip too much bone too much conga too much cuatro to two step got too many piano keys in between her teeth, it got too much clave too much hand clap got too much salsa to sit still it be an anxious child wanting to make Play-Doh out of concrete English be too neat for her kind of wonderful. her words spill in conversation between women whose hands are all they got sometimes our hands are all we got and accents that remind us that we are still bomba, still plena you say "wepa" and a stranger becomes your hermano, you say "dale" and a crowd becomes a family reunion. my mother's tongue is a telegram from her mother decorated with the coqui's of el campo so even when her lips can barely stretch themselves around english, her accent is a stubborn compass always pointing her towards home.

P27 ?An unsung hero of the civil rights movement?

In the middle of the 16th century, a talented young anatomist named Andre?as Vesalius made a shocking discovery:the most famous human anatomy texts in ?the world were wrong.They not only failed to account for many details of the human body, they also described the organs of apes and other mammals.While Vesalius knew he was right, announcing these errors would mean challenging Galen of Pergam?on–the most renowned physician ?in medical history. But who was this towering figure? And why did doctors working more than 1,600 years later so revere and fear him? Born in 129 CE,Galen left home as a teen to scour the Mediterranean for medical wisdom. He returned home a gifted surgeon with a passion for anatomy?and a penchant for showmanship. He gleefully entered public anatomy ?contests,eager to show up his fellow physicians.In one demonstration, he caused a pig to lose its voice by tying off one of its nerves. In another, he disemboweled a monkey and challenged his colleagues to repair it.When they couldn’t, he did. These grizzly feats won him a position as ?surgeon to the city’s gladiators.Eventually, he would leave the arena ?to become the personal physician?to four Roman Emperors. While his peers debated symptoms and ?their origins,Galen obsessively studied anatomy. He was convinced that each organ had a specific function.Since the Roman government largely ?prohibited working with human cadavers, Galen conducted countless dissections ?of animals instead.Even with this constraint, his exhaustive investigations yielded ?some remarkably accurate conclusions. One of Galen’s most important ?contributions?was the insight that the brain, not the heart, controlled the body.He confirmed this theory by opening the cranium of a living cow. By applying pressure to different ?parts of the brain,he could link various regions ?to specific functions. Other experiments allowed him to ?distinguish sensory from motor nerves, establish that urine was ?made in the kidneys, and deduce that respiration was controlled by muscles and nerves. But these wild experiments also produced ?extraordinary misconceptions. Galen never realized that blood cycles continuously throughout the body.Instead, he believed the liver constantly produces an endless supply of blood, which gets entirely depleted on its ?one-way trip to the organs. Galen is also credited with solidifying ?the popular theory of the Four Humours. Introduced by Hippocrates ?centuries earlier, this misguided hypothesis attributed most medical problems?to an imbalance in four bodily fluids ?called humours.To correct the balance of these fluids, ?doctors employed dangerous treatments?like bloodletting and purging. Informed by his poor understanding ?of the circulatory system, Galen was a strong proponent ?of these treatments,despite their sometimes lethal ?consequences.Unfortunately, Galen’s ego ?drove him to believe that?all his discoveries were ?of the utmost importance.He penned treatises on everything from anatomy to nutrition to bedside manner, meticulously cataloguing his writings ?to ensure their preservation. Over the next 13 centuries,Galen’s prolific collection dominated ?all other schools of medical thought.His texts became the standard works ?taught to new generations of doctors,who in turn, wrote new essays extolling ?Galen’s ideas. Even doctors who actually dissected human cadavers?would bafflingly repeat Galen’s mistakes, despite seeing clear evidence to the contrary.?Meanwhile, the few practitioners bold ?enough to offer conflicting opinions?were either ignored or ridiculed.For 1,300 years, Galen’s legacy ?remained untouchable–until renaissance anatomist Vesalius ?spoke out against him. As a prominent scientist and lecturer, his authority influenced many young ?doctors of his time. But even then, it took another hundred years?for an accurate description ?of blood flow to emerge, and two hundred more for the theory ?of the Four Humours to fade.Hopefully, today we can reap the benefits of Galen’s experiments?without attributing equal credence ?to his less accurate ideas. But perhaps just as valuableis the reminder that science is an ?ever-evolving process,which should always?place evidence above ego.

P28? ?Ancient Rome’s most notorious doctor

In the middle of the 16th century, a talented young anatomist named Andreas Vesalius made a shocking discovery: the most famous human anatomy texts in the world were wrong. They not only failed to account for many details of the human body, they also described the organs of apes and other mammals. While Vesalius knew he was right, announcing these errors would mean challenging Galen of Pergamon– the most renowned physician in medical history. But who was this towering figure? And why did doctors working more than 1,300 years later so revere and fear him? Born in 129 CE, Galen left home as a teen to scour the Mediterranean for medical wisdom. He returned home a gifted surgeon with a passion for anatomy and a penchant for showmanship. He gleefully entered public anatomy contests, eager to show up his fellow physicians. In one demonstration, he caused a pig to lose its voice by tying off one of its nerves. In another, he disemboweled a monkey and challenged his colleagues to repair it. When they couldn’t, he did. These grizzly feats won him a position as surgeon to the city’s gladiators. Eventually, he would leave the arena to become the personal physician to four Roman Emperors. While his peers debated symptoms and their origins, Galen obsessively studied anatomy. He was convinced that each organ had a specific function. Since the Roman government largely prohibited working with human cadavers, Galen conducted countless dissections of animals instead. Even with this constraint, his exhaustive investigations yielded some remarkably accurate conclusions. One of Galen’s most important contributions was the insight that the brain, not the heart, controlled the body. He confirmed this theory by opening the cranium of a living cow. By applying pressure to different parts of the brain, he could link various regions to specific functions. Other experiments allowed him to distinguish sensory from motor nerves, establish that urine was made in the kidneys, and deduce that respiration was controlled by muscles and nerves. But these wild experiments also produced extraordinary misconceptions. Galen never realized that blood cycles continuously throughout the body. Instead, he believed the liver constantly produces an endless supply of blood, which gets entirely depleted on its one-way trip to the organs. Galen is also credited with solidifying the popular theory of the Four Humours. Introduced by Hippocrates centuries earlier, this misguided hypothesis attributed most medical problems to an imbalance in four bodily fluids called humours. To correct the balance of these fluids, doctors employed dangerous treatments like bloodletting and purging. Informed by his poor understanding of the circulatory system, Galen was a strong proponent of these treatments, despite their sometimes lethal consequences. Unfortunately, Galen’s ego drove him to believe that all his discoveries were of the utmost importance. He penned treatises on everything from anatomy to nutrition to bedside manner, meticulously cataloguing his writings to ensure their preservation. Over the next 13 centuries, Galen’s prolific collection dominated all other schools of medical thought. His texts became the standard works taught to new generations of doctors, who in turn, wrote new essays extolling Galen’s ideas. Even doctors who actually dissected human cadavers would bafflingly repeat Galen’s mistakes, despite seeing clear evidence to the contrary. Meanwhile, the few practitioners bold enough to offer conflicting opinions were either ignored or ridiculed. For 1,300 years, Galen’s legacy remained untouchable– until renaissance anatomist Vesalius spoke out against him. As a prominent scientist and lecturer, his authority influenced many young doctors of his time. But even then, it took another hundred years for an accurate description of blood flow to emerge, and two hundred more for the theory of the Four Humours to fade. Hopefully, today we can reap the benefits of Galen’s experiments without attributing equal credence to his less accurate ideas. But perhaps just as valuable is the reminder that science is an ever-evolving process, which should always place evidence above ego.

P29??Aphasia - The disorder that makes you lose your words

Language is an essential part of our lives that we often take for granted. With it, we can communicate our thoughts and feelings, lose ourselves in novels, send text messages, and greet friends. It's hard to imagine being unable to turn thoughts into words. But if the delicate web of language networks in your brain became disrupted by stroke, illness, or trauma, you could find yourself truly at a loss for words. This disorder, called aphasia, can impair all aspects of communication. People who have aphasia remain as intelligent as ever. They know what they want to say, but can't always get their words to come out correctly. They may unintentionally use substitutions called paraphasias, switching related words, like saying "dog" for "cat," or words that sound similar, such as "house" for "horse." Sometimes, their words may even be unrecognizable. There are several types of aphasia grouped into two categories: fluent, or receptive, aphasia and non-fluent, or expressive, aphasia. People with fluent aphasia may have normal vocal inflection but use words that lack meaning. They have difficulty comprehending the speech of others and are frequently unable to recognize their own speech errors. People with non-fluent aphasia, on the other hand, may have good comprehension but will experience long hesitations between words and make grammatical errors. We all have that tip-of-the-tongue feeling from time to time when we can't think of a word, but having aphasia can make it hard to name simple, everyday objects. Even reading and writing can be difficult and frustrating. So how does this language loss happen? The human brain has two hemispheres. In most people, the left hemisphere governs language. We know this because in 1861, the physician Paul Broca studied a patient who lost the ability to use all but a single word, "tan." During a postmortem study of that patient's brain, Broca discovered a large lesion in the left hemisphere now known as Broca's area. Scientists today believe that Broca's area is responsible in part for naming objects and coordinating the muscles involved in speech. Behind Broca's area is Wernicke's area near the auditory cortex. That's where the brain attaches meaning to speech sounds. Damage to Wernicke's area impairs the brain's ability to comprehend language. Aphasia is caused by injury to one or both of these specialized language areas. Fortunately, there are other areas of the brain which support these language centers and can assist with communication. Even brain areas that control movement are connected to language. FMRI studies found that when we hear action words, like "run" or "dance," parts of the brain responsible for movement light up as if the body was actually running or dancing. Our other hemisphere contributes to language, too, enhancing the rhythm and intonation of our speech. These non-language areas sometimes assist people with aphasia when communication is difficult. So how common is aphasia? approximately 1 million people in the U.S. alone have it, with an estimated 80,000 new cases per year. About one-third of stroke survivors suffer from aphasia making it more prevalent than Parkinson's disease or multiple sclerosis, yet less widely known. There is one rare form of aphasia called primary progressive aphasia, or PPA, which is not caused by stroke or brain injury, but is actually a form of dementia in which language loss is the first symptom. The goal in treating PPA is to maintain language function for as long as possible before other symptoms of dementia eventually occur. However, when aphasia is acquired from a stroke or brain trauma, language improvement may be achieved through speech therapy. Our brain's ability to repair itself, known as brain plasticity, permits areas surrounding a brain lesion to take over some functions during the recovery process. Scientists have been conducting experiments using new forms of technology, which they believe may encourage brain plasticity in people with aphasia. Meanwhile, many people with aphasia remain isolated, afraid that others won't understand them or give them extra time to speak. By offering them the time and flexibility to communicate in whatever way they can, you can help open the door to language again, moving beyond the limitations of aphasia.

P30??Are food preservatives bad for you

Food doesn't last. In days, sometimes hours, bread goes moldy, apple slices turn brown, and bacteria multiply in mayonnaise. But you can find all of these foods out on the shelf at the grocery store, hopefully unspoiled, thanks to preservatives. But what exactly are preservatives? How do they help keep food edible and are they safe? There are two major factors that cause food to go bad: microbes and oxidation. Microbes like bacteria and fungi invade food and feed off its nutrients. Some of these can cause diseases, like listeria and botulism. Others just turn edibles into a smelly, slimy, moldy mess. Meanwhile, oxidation is a chemical change in the food's molecules caused by enzymes or free radicals which turn fats rancid and brown produce, like apples and potatoes. Preservatives can prevent both types of deterioration. Before the invention of artificial refrigeration, fungi and bacteria could run rampant in food. So we found ways to create an inhospitable environment for microbes. For example, making the food more acidic unravels enzymes that microbes need to survive. And some types of bacteria can actually help. For thousands of years, people preserved food using bacteria that produce lactic acid. The acid turns perishable vegetables and milk into longer lasting foods, like sauerkraut in Europe, kimchi in Korea, and yogurt in the Middle East. These cultured foods also populate your digestive track with beneficial microbes. Many synthetic preservatives are also acids. Benzoic acid in salad dressing, sorbic acid in cheese, and propionic acid in baked goods. Are they safe? Some studies suggest that benzoates, related to benzoic acid, contribute to hyperactive behavior. But the results aren't conclusive. Otherwise, these acids seem to be perfectly safe. Another antimicrobial strategy is to add a lot of sugar, like in jam, or salt, like in salted meats. Sugar and salt hold on to water that microbes need to grow and actually suck moisture out of any cells that may be hanging around, thus destroying them. Of course, too much sugar and salt can increase your risk of heart disease, diabetes, and high blood pressure, so these preservatives are best in moderation. Antimicrobial nitrates and nitrites, often found in cured meats, ward off the bacteria that cause botulism, but they may cause other health problems. Some studies linking cured meats to cancer have suggested that these preservatives may be the culprit. Meanwhile, antioxidant preservatives prevent the chemical changes that can give food an off-flavor or color. Smoke has been used to preserve food for millennia because some of the aromatic compounds in wood smoke are antioxidants. Combining smoking with salting was an effective way of preserving meat before refrigeration. For antioxidant activity without a smoky flavor, there are compounds like BHT and tocopherol, better known as vitamin E. Like the compounds in smoke, these sop up free radicals and stave off rancid flavors that can develop in foods like oils, cheese, and cereal. Other antioxidants like citric acid and ascorbic acid help cut produce keep its color by thwarting the enzyme that causes browning. Some compounds like sulfites can multitask. They're both antimicrobials and antioxidants. Sulfites may cause allergy symptoms in some people, but most antioxidant preservatives are generally recognized as safe. So should you be worried about preservatives? Well, they're usually near the end of the ingredients list because they're used in very small amounts determined by the FDA to be safe. Nevertheless, some consumers and companies are trying to find alternatives. Packaging tricks, like reducing the oxygen around the food can help, but without some kind of chemical assistance, there are very few foods that can stay shelf stable for long.