Simon Whistler explained why Mexican jumping beans jump in an episode of his incredibly interesting and informative web series Today I Found Out.
Not actually a bean at all (beans being seeds from the family Fabaceae, also known as Leguminosae), the Mexican Jumping Bean is the seed pod of a shrub called Sebastiania pavoniana that has been implanted with larva from the Laspeyresia saltitans (also called Cydia deshaisiana) moth.
