As you may recognize from the commercials, Jell-O is basically a brand name for gelatin (with some type of sweetener, flavoring, and coloring).

Gelatin, which is processed form of collagen, is the ingredient of Jell-O that gives it that semisolid texture when it is cooled down. You can imagine gelatin as three amino acid chains (polypeptide chains) that wind around each other in a helical structure. Specifically, the three chains wrap around each other in a triple helix structure. (To visualize this, imagine - or look up - the "classic" double helix structure of DNA, and then imagine a third chain also winding around with it.) Much like in DNA, the three chains are weakly bound together through interactions between the different parts of the amino acids.

When the gelatin is heated in water, those weak bonds break and the polypeptide chains become free and disordered. When the gelatin solution cools, the chains begin to re-associate with each other. However, instead of reforming a perfect triple helix when the chains associate with other chains, drops of liquid get trapped between the polypeptide chains. Thus Jell-O is simply this network of trapped liquid in protein chains.

Jell-O is made out of something called gelatin (it is pronounced like jell-atin). Gelatin is a processed form of a kind of protein found in animals. If you don't already know, every living thing is made almost entirely out of proteins (and lots of water!).

Gelatin has a very particular structure: It is made out of three smaller fibers that are wrapped around each other like a long rope. The smaller fibers like to be attached - that's what keeps them wrapped around each other. When you heat up some gelatin mixed into water, the water molecules bump into the fibers a lot and cause them to come apart. That is the liquid form of jell-o.

When the jell-o cools down again, the fibers come back together...sort of. It turns out that when the fibers come back together, they end up being tangled up. That's because, though the fibers want to be attached to each other, they don't care if they are attached to the same fiber all the way down their length (the way they started) or lots of different fibers. They just want to touch as many other fibers as possible. In your body, I think there are other proteins that make sure all the fibers combine properly but in jell-o, all the different fibers get tangled into a big net. The water gets trapped between all the fibers and the result is something that acts a little bit like a solid. It is usually called a "gel", though.