Returning to our discussion of Jell-O, what happened at a molecular scale that resulted in the formation of hydrogel? Collagen protein is extracted from skin, crushed bones, connective tissue and other animal parts, and then is partially hydrolyzed to make gelatin, a mixture of peptides and proteins. When gelatin is dissolved in hot water, the long protein polymer chains move freely in solution. As the solution cools, the motion of the polymer chains slows down. Polymer chains encounter each other and become entangled. As the degree of entanglement increases, water is trapped and immobilized. As a result, the material behaves in some ways like a liquid and other ways like a solid.