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If you fell into a swimming pool full of Jello, would you be able to swim to the other side? — LM, Charlottesville, VA


That question certainly takes me back. Jello was one of the big mysteries of my childhood. At school, I was taught that there are exactly three phases of matter: solid, liquid, and gas. According to my teachers, everything fit neatly into one of those categories. But what about Jello?! To which of these black & white categories did Jello belong? I asked my teachers, and just about everyone else, but no one would tell me. I felt like a subversive, a rebel, and a troublemaker.

What I didn’t know is that rigid categories often oversimplify the world and neglect wonderfully interesting possibilities. 30 years later, I can tell you that Jello actually spans the range from liquid to solid, depending mostly on its concentration. When it’s sufficiently dilute, it’s definitely a thin liquid. When it’s highly concentrated, it’s definitely a stiff, elastic solid. And then there’s the complicated in between.

Reaction:
The basic distinction between a solid and a liquid is how a material responds to stress—how it responds to having its different parts pushed in different directions. If a material flows when stressed and ultimately rearranges itself so as to eliminate the stress, then it’s technically a liquid. If instead it deforms temporarily and then springs back elastically when the stress is removed, then it’s technically a solid.

But the distinction isn’t perfectly sharp; it depends on how patient you are. Some materials respond elastically to stresses of short duration, but flow liquid-like when stressed for long periods of time. Jello is just such a material. When you smack Jello with a spoon, it jiggles elastically; it behaves like a rubbery solid. But if you squeeze Jello for hours in a stack of serving plates in your refrigerator, it gradually deforms; it behaves like a thick liquid. Officially, a material that flows eventually is a liquid and one that never, ever flows is a solid, but who wants to wait that long?

With that long introduction, it’s time to answer your question. Whether or not you can swim across the pool of Jello answer depends on how concentrated the Jello is. If the Jello is sufficiently dilute that it behaves as a liquid, you’ll be able to swim through it. It may take some time, but you’ll eventually get to the other side.
But if the Jello is so concentrated that it behaves as a solid, the best you can do is to dig your way across. The pool might as well be full of dirt.

And between the two extremes of concentration lie a range of possibilities and levels of patience. As the Jello gets more and more concentrated, the time it takes to flow lengthens and the patience you’ll need to swim across the pool increases. Eventually, you’ll need infinite patience, and then it’s time to beginning digging through what is effectively solid Jello.