What causes the Mexican Jumping Bean to jump?

The bean moth lays an egg within the seed pod, the egg hatches which produces a larva or caterpillar. When the bean is heated by sunlight or the heat from your hand the larva shifts it’s weight causing the jumping action.

[youtube=http://www.youtube.com/watch?v=hQKMkBtRGB8]

Thanks to W.P. Armstrong for this great picture of the jumping bean. http://waynesword.palomar.edu/mrwolfia.htm