Mexican jumping beans are a nifty amusement often found at toy stores and bazaars. They are sold in groups of 5-10 to ensure at least one "live" bean. The beans appear to jump around under their own power, at least for a few days before they die.
Mexican jumping beans are not actually beans. They are seeds of the jumping bean shrub (Sebastiana pavoniana) which can be found in parts of Mexico. The beans do not jump by themselves; the jumping motion is caused by a moth larva (Cydia saltitans) which has infested the seed.