Jelly babies made with gelatin swell in cold water, while those made with gum arabic, agar, pectin or starch fall apart. Contrary to popular belief, the fact that gelatin jelly babies swell up has nothing to do with osmosis. In osmosis, water passes through a membrane to balance out different concentrations of dissolved particles, such as salt or sugar. However, jelly babies are not wrapped in a membrane. Gelatin is able to absorb large quantities of water because the water molecules attach themselves to particular structures of the protein molecules in the gelatin, greatly increasing their volume. Chemists call this process geletifaction.
After three days in water, a jelly baby is nearly twice its original length-but its flavor leaves much to be desired.