Why You Can’t Bring A Jar Of Peanut Butter On A Plane—And Other Illusive Liquids

Peanut butter’s thick, sticky spread is not a solid, but a liquid. Explore the many curious properties of fluid materials that can be tricky to grasp.

The following is an excerpt from Liquid Rules: The Delightful and Dangerous Substances That Flow Through Our Lives by Mark Miodownik.

Peanut butter, honey, pesto sauce, toothpaste, and, most painfully, a bottle of single-malt whiskey—these are just some of the liquids I’ve had confiscated at airport security. I inevitably lose the plot in such situations. I say things like “I want to see your supervisor” or “Peanut butter is not a liquid,” even though I know it is. Peanut butter flows and assumes the shape of its container—that is what liquids do—and so peanut butter is one. Even so, it just infuriates me that in a world full of “smart” technology, airport security still can’t tell the difference between a liquid spread and a liquid explosive.

a book cover of "liquid rules" by mark Miodownik. it is black and features different circles that showcase forms of liquid, like a volcano and lava, and water and as a wave

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Liquid Rules: The Delightful and Dangerous Substances That Flow Through Our Lives


Although bringing more than 3.4 ounces of liquid through security has been banned since 2006, our detection technology has not improved much in that time. X-ray machines can see through your luggage to detect objects. In particular they alert security to suspicious shapes: distinguishing guns from hairdryers, and knives from pens. But liquids don’t have a shape. They just take the form of whatever is containing them. Airport scanning technology can also detect density and a range of chemical elements. But here again they run into problems. The molecular makeup of the explosive nitroglycerin, for instance, is similar to peanut butter’s. They’re both made from carbon, hydrogen, nitrogen, and oxygen—and yet one is a liquid explosive while the other is just, well, delicious. There are an enormous number of dangerous toxins, poisons, bleaches, and pathogens that are incredibly difficult to distinguish from more innocent liquids in a quick and reliable manner. And it is this line of argument, which I’ve heard from many security guards (and their supervisors), that usually persuades me to agree—begrudgingly—that my peanut butter, or one of the other liquids I seem to regularly forget to take out of my carry-on luggage, is a significant risk.

The molecular makeup of the explosive nitroglycerin, for instance, is similar to peanut butter’s.

Liquids are the alter ego of dependable solid stuff. Whereas solid materials are humanity’s faithful friend, taking on the permanent shapes of clothes, shoes, phones, cars, and indeed airports, liquids are fluid; they will take on any shape, but only while contained. When they’re not contained, they are always on the move, seeping, corroding, dripping, and escaping our control. If you put a solid material somewhere, it stays there—barring forcible removal—often doing something very useful, like holding up a building or supplying electricity for a whole community. Liquids, on the other hand, are anarchic: they have a knack for destroying things. In a bathroom, for instance, it is a constant battle to keep water from seeping into cracks and pooling under the floor, where it gets up to no good, rotting and undermining the wooden joists; on a smooth tile floor, water is the perfect slip hazard, causing an enormous number of injuries; and when it gathers in the corners of the bathroom, it can harbor black slimy fungus and bacteria, which threaten to infiltrate our bodies and make us sick. And yet, despite all this treachery, we love the stuff; we love bathing in water, and showering in it, drenching the whole body. And what bathroom is complete without a cornucopia of bottled liquid soaps, shampoos, and conditioners, jars of cream, and tubes of toothpaste? We delight in these miraculous liquids and yet we worry about them too: Are they bad for us? Do they cause cancer? Do they ruin the environment? With liquids, delight and suspicion go hand in hand. They are duplicitous by nature, neither a gas nor a solid but something in between, something inscrutable and mysterious.

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The Fluids That Flow Through Our Lives

Take mercury, for instance, which has delighted and poisoned humanity for thousands of years. As a child I used to play with liquid mercury, flicking it around on tabletops, fascinated by its otherworldliness, until I was made aware of its toxicity. But in many ancient cultures mercury was thought to prolong life, heal fractures, and maintain good health. It’s not clear why it was held in such regard-perhaps because it is special in being the only pure metal that is liquid at room temperature. The first emperor of China, Qin Shi Huang, took mercury pills for his health but died at 39, probably as a result. Even so, he was buried in a tomb full of rivers of flowing mercury. The ancient Greeks used mercury in ointments, and alchemists believed that a combination of mercury and another elemental substance, sulfur, formed the basis of all metals—a perfect balance between mercury and sulfur creating gold. This was the origin of the erroneous belief that different metals could be transmuted into gold if mixed in the right proportions. While that proved to be the stuff of legend, gold does dissolve in mercury. If you heat up that liquid once it has absorbed the metal, it will evaporate, leaving behind a solid lump of gold. For most ancient people, that process was indistinguishable from magic.

Mercury isn’t the only liquid that can consume another substance and contain it within itself. Add salt to water and it will soon disappear—the salt is somewhere, but where has it gone? Yet if you do the same with oil, the salt just sits there. Why? Liquid mercury may absorb solid gold, but it rejects water. Why is that? Water absorbs gases, including oxygen, and if it didn’t, we would live in a very different world—it is the oxygen dissolved in water that allows fish to breathe. And although water can’t carry enough oxygen for humans to breathe, other liquids can. There’s a type of oil—perfluorocarbon liquid—that is very unreactive chemically and electrically. It is so inert that you can put your cellphone in a beaker of perfluorocarbon liquid and it will continue to operate normally. Perfluorocarbon liquid can also absorb oxygen in such high concentrations that it is breathable by humans. This sort of liquid breathing—breathing liquid instead of air—has many possible uses, the most important of which is to treat premature babies suffering from respiratory-distress syndrome.

Liquids are anarchic: they have a knack for destroying things.

Still, it is liquid water that has the ultimate life-giving property. This is because it can dissolve not just oxygen, but many other chemicals too, including carbon-based molecules, and so provides the aqueous environment required for the emergence of life—for new organisms to spontaneously come into existence. Or at least that is the theory. And it is why, when scientists look for life on other planets, they look for liquid water. But liquid water is rare in the universe. It’s possible that Europa, one of the moons of Jupiter, might have oceans of liquid water below its icy crust. There might also be liquid water on Enceladus, one of the moons of Saturn. But Earth is the only body in the solar system where a lot of water is readily available on the surface.

A peculiar set of circumstances on our planet has made possible the surface temperatures and pressures that can sustain liquid water. In particular, if it weren’t for Earth’s molten-metal liquid core, which creates the magnetic field that protects us from the solar wind, all our water would most likely have disappeared billions of years ago. In short, on our planet, liquid begat liquid, and that led to life.

But liquids are destructive too. Foams feel soft because they are easily compressed; if you jump on to a foam mattress, you’ll feel it give beneath you. Liquids don’t do this; instead they flow-with one molecule moving into the space freed up by another molecule. You see this in a river, or when you turn on a tap, or if you use a spoon to stir your coffee. When you jump off a diving board and hit a body of water, the water has to flow away from you. But the flowing takes time, and if your speed of impact is too great, the water won’t be able to flow away fast enough, and so it pushes back at you. It’s that force which stings your skin as you belly-flop into a pool, and makes falling into water from a great height like landing on concrete. The incompressibility of water is also why waves can exert such deadly power, and in the case of tsunamis, why they can demolish buildings and cities, tossing cars around as if they were driftwood. For instance, the Indian Ocean earthquake in 2004 triggered a series of tsunamis, killing 230,000 people across fourteen countries. It was the eighth-worst natural disaster ever recorded.

Another dangerous property of liquids is their ability to explode. When I began my Ph.D. at Oxford University, I had to prepare small specimens for the electron microscope. This involved cooling a liquid called an electropolishing solution to a temperature of -4°F. The liquid was a mixture of butoxyethanol, acetic acid, and perchloric acid. Another Ph.D. student in the lab, Andy Godfrey, showed me how to do this, and I thought I’d gotten the hang of it. But after a few months Andy noticed that I often let the temperature of the solution rise while electropolishing. “I wouldn’t do that,” he said, raising his eyebrows as he peered over my shoulder one day. When I inquired why, he pointed me toward the lab manual of chemical hazards:

Perchloric acid is a corrosive acid and destructive to human tissue. Perchloric acid can be a health hazard if inhaled, ingested, or splashed on skin or eyes. Once heated above room temperature or used at concentrations above 72 percent (any temperature), perchloric acid becomes a strong oxidizing acid. Organic materials are especially susceptible to spontaneous combustion if mixed or contacted with perchloric acid. Perchloric acid vapours may form shock-sensitive perchlorates in ventilation system ductwork.


In other words, it can explode.

Upon inspection of the lab I found many similarly transparent colorless liquids, most of which were indistinguishable from one another. We used hydrofluoric acid, for instance, which, apart from being an acid that can eat its way through concrete, metals, and flesh, is also a contact poison that interferes with nerve function. This has an insidious consequence—namely, that you can’t feel the acid as it’s burning you. Accidental exposure can easily go unnoticed as the acid continues to eat its way through your skin.

Alcohol too fits into the category of poison. It may be poisonous only in high doses, but it has killed many more people than hydrofluoric acid has. Yet alcohol plays an enormous role in societies and cultures around the globe, having historically been used as an antiseptic, an antitussive, an antidote, a tranquilizer, and a fuel. Alcohol’s main attraction is that it depresses the nervous system—it’s a psychoactive drug. Many people can’t function without their daily glass of wine, and most social functions revolve around places where alcohol is served. We (rightly) may not trust these liquids, but we love them anyway.

We feel alcohol’s physiological effects as it’s absorbed into the bloodstream. The thumping of our heartbeat is a constant reminder of blood’s role in the body, and its need to constantly circulate: we run thanks to the power of a pump, and when the pumping stops, we die. Of all the liquids in the world, surely blood is one of the most vital. Fortunately, the heart can now be replaced, bypassed, and plumbed in and out of the body. Blood itself can be added and removed, stored, shared, frozen, and revived. And indeed, without our blood banks, every year millions of people undergoing surgery, injured in war zones, or involved in traffic accidents would die.

But blood can be contaminated with infections such as HIV and hepatitis, and so it can harm as well as heal. Thus, we must take into consideration the duplicitous nature of blood, as of all liquids. The important question is not whether a particular liquid can be trusted or not, is good or bad, is healthy or poisonous, is delicious or disgusting, but rather whether we understand it enough to harness it.

With liquids, delight and suspicion go hand in hand. They are duplicitous by nature, neither a gas nor a solid but something in between, something inscrutable and mysterious.

There is no better way to illustrate the power and delight we gain from controlling liquids than by taking a look at those involved in the flight of an airplane and the experience of the passengers onboard. And so that is what this book is about, a transatlantic flight, and all the strange and wonderful liquids that play a part in it. I took this flight because I had not blown myself up while completing my Ph.D., but had instead continued to do materials science research and eventually become director of the Institute of Making, at University College London. There, part of our research involves understanding how liquids can masquerade as solids. For instance, the tar from which roads are made is, like peanut butter, a liquid, even though it gives the impression of being a solid. Our research has led to invitations to fly to conferences all around the world, and this book is an account of one such trip, from London to San Francisco.

The flight is described through the language of molecules, heartbeats, and ocean waves. My aim is to unlock the mysterious properties of liquids and how we have come to rely on them. The flight takes us over the volcanoes of Iceland, the frozen expanse of Greenland, the lakes dotted around Hudson Bay, and then south to the coastline of the Pacific Ocean. This canvas is big enough to accommodate a discussion of liquids from the scale of oceans down to droplets in the clouds, along with the curious liquid crystals in the onboard entertainment system, the beverages served by the flight attendants, and of course, the aviation fuel that keeps a plane in the stratosphere.

In each chapter I consider a different part of the flight, and the qualities of liquids that made it possible: their ability to combust, to dissolve, or to be brewed, to name a few. I show how wicking, droplet formation, viscosity, solubility, pressure, surface tension, and many other strange properties of liquids can allow us to fly around the globe. And in doing so I reveal why liquids flow up a tree but down a hill, why oil is sticky, how waves can travel so far, why things dry, how liquids can be crystals, how not to poison yourself making hooch, and most important perhaps, how to make the perfect cup of tea. So please, come fly with me—I can promise you a strange and marvelous trip.

Excerpted from Liquid Rules by Mark Miodownik. Copyright © 2019 by Mark Miodownik. Reprinted by permission of Houghton Mifflin Harcourt Publishing Company. All rights reserved.

Meet the Writer

About Mark Miodownik

Mark Miodownik is author of Stuff Matters: Exploring the Marvelous Materials that Shape our Man-Made World and Liquid Rules: The Delightful and Dangerous Substances That Flow Through Our Lives. He’s also director of the Institute of Making and professor of materials and society at University College London in London, England.

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