http://dopamine.chem.umn.edu/chempedia/index.php/Jello
It must be collagen that does it. That's pretty cool. I wish I knew that in grade 7 as a counter to all the smug co-7-year-olds who tried to turn me off my jello by telling me that jello was made with ground-up horse bones. >.< Oh well. Something to tell my kids if I ever have any...