Mexican jumping beans jump because of a larva inside

After the egg has hatched, the larva eats away the inside of the bean, making a hollow for itself. It attaches itself to the bean with many silk threads.

The larva may live for months inside the bean with varying periods of dormancy. If the larva has adequate conditions such as moisture, it will live long enough to go into a pupal stage. Normally in the spring, the moth will force its way out of the bean through a round "trap door", leaving behind the pupal casing. The small, silver and gray-colored moth will live for only a few days.

The beans jump as a survival measure in order to protect the larvae from the heat, which can cause them to dry out. The ultraviolet rays from the sun stimulate them to jump, even in cool temperatures, but leaving them in the sun for extended periods will dehydrate and kill them.

http://en.wikipedia.org/wiki/Mexican_jumping_bean