The special chemical properties of gelatin make it possible for home cooks to turn flavored powder into a jiggly dessert in the shape of their choice. At room temperature, collagen, which is what gelatin is made from, consists of a bunch of triple helices. These helices consist of three strands of amino acids, called polypeptide chains, wrapped around one another, with weak bonds holding them together. When boiling water is poured over dried gelatin these bonds dissolve and the helices fall apart, leaving a puddle of loose polypeptide chains.

Adding cold water to the liquid and chilling it in the fridge rebuilds the bonds and the triple helix structure, and any water that was added gets absorbed into the blob. The chains form a kind of net that catches any excess water in its gaps. The strength of the triple helix structure combined with these water pockets is what makes Jell-O jiggle while still holding the shape of the vessel it was set in.