Peanut butter is a liquid.  So are cats and sleeping babies.

This article was originally posted by The conversation.

These TSA requirements are drilled into every frequent flyer’s head: Liquids can only be carried in containers that have a volume of 3.4 ounces (100 milliliters) or less.

But when the TSA recently confiscated a jar of Jif under this rule, peanut butter lovers were up in arms. Some security skeptics may suspect that the hungry officers just wanted to make their own peanut butter and jelly sandwiches. The TSA, however, maintains that peanut butter is a liquid, and a full-size jar of Jif exceeds the 3.4-ounce limit.

Just like the beloved of Americans legume-based sandwich ingredient, the story, and the outrage it inspired, spread. However, I am a mechanical engineer who studies fluid flows, and the TSA action made sense to me. By the scientific definition, peanut butter is in fact a liquid.

To define a liquid, we must first define a fluid. Any material that flows continuously when a shear force is applied is a fluid. Think of a shear force as a shearing action through a substance that causes it to flow continuously. For example, moving your arm causes the surrounding air to change shape, or warp, to use the physical term, and flow out of the way. The same thing happens with water when your arm takes a swimming stroke.

There are many types of fluids. Some act very predictably and move smoothly, like air and water. These are called newtonian fluids, named after Sir Isaac Newton. Scientifically, a Newtonian fluid is one in which the shear force varies in direct proportion to the stress it exerts on the material, known as shear stress. The resistance of a Newtonian fluid to fluid flow, that is, its viscosity, is constant at a given temperature.

Other types of fluids do not move as smoothly and easily. Some, such as peanut butter, may require minimal shear or shear to begin to flow, and this shear may vary non-linearly with shear stress. Imagine that you are stirring a jar of peanut butter. If you stir very fast, with more cutting force, the PB becomes more fluid; if you stir slowly, the PB becomes stiff. These types of fluids are called non-Newtonian fluids. Peanut butter can stick more than flow; you might consider this move with a chunkier style.

Peanut butter is actually a great example of a non-Newtonian fluid: it doesn’t flow as easily as air or water, but it will flow if enough force is applied, such as when a knife spreads it across bread. The ease with which it flows will also depend on the temperature; you may have experienced peanut butter drippings on hot toast.

Our everyday lives, but not our carry-on luggage on the plane, are full of substances that are unexpected fluids. In general, if it can flow, it is a fluid. And it will eventually take the shape of its container.

Some amazing liquids are peanut butter’s kitchen neighbors, like whipped cream, mayonnaise, and cookie dough. You will find others in the bathroom, toothpaste, for example. The natural world harbors many strange fluids, including wash, mudavalanche snow and quicksand.

The gravel can be considered fluid. Individual particles are solid, but a collection of gravel particles can be poured and fill a container; it is what is called a “granular fluid”, because it has fluid-like properties. The same can be said for cereal poured out of a box or sugar in a bowl.

You might even consider a cat lying in the sun, flattened and filling its container skin, to be a fluid. Sleepy and relaxed dogs; squirrels; and even sleepy babies can meet the definition of fluid.

Now, you might be quibbling: But the TSA didn’t call peanut butter a liquid; He said it’s a liquid!

Fluids fall into two general categories: gases and liquids. Both gases and liquids can be deformed and poured into containers, and they will take the shape of their container. But gases can be compressed, while liquids can’t, at least not easily.

Peanut butter can be poured into a container, at which point it deforms or takes the shape of that container. And every 5-year-old knows that peanut butter doesn’t compress: when you squash peanut butter and peanut butter cookies, the peanut butter doesn’t squash into a smaller volume. No, it squirts out the sides and on the hands.

So, the verdict on peanut butter: liquid delicious. If you plan to make a PB&J sandwich mid-flight, count on bringing 3.4 ounces of liquid peanut butter or less. And the same goes for its liquid cousin, gelatin.