Jell-O, which is a brand name for gelatin, does not melt after hardening because of its main ingredient, collagen. When dissolved in water, Jell-O becomes a tangle of long strands of amino acids that are capable of holding the water, thus seemingly solidifying it.
The collagen used in making Jell-O comes from the skins and bones of pigs, cows and horses. These parts are boiled in water to extract the collagen, which is a protein. The powder form available in supermarkets is dried gelatin. Not all fruits can be used when making gelatin desserts. Fresh pineapple, kiwi, figs and papaya prevent gelatin from setting.