The mexican jumping bean "jumps" because a caterpillar is banging its head on the side of a seed. The whole thing starts when a moth Cydia saltitans deposits its eggs on the flower of a Mexican shrub. When the eggs hatch the caterpillars burrow into the developing seeds of the flower. The voracious little squirmlings eat all of the soft interior of the seed, leaving only the hard hull, then build a cozy little web on the inside.
The actual jumping of the seed results when the caterpillar grabs onto the web with its front claws and jerks its body violently, actually hitting its head against the side of the hull. This is apparently a self-defense mechanism and is very effective in scaring away bird and other animals that might being thinking of making the seed their lunch. Hmmmm..do you think the term "butterflies in my stomach" was somehow derived from these critters? If a creature ever did swallow one of the inhabited beans, the action of the bean would increase, as warmth makes the caterpillar move even more. There is also speculation that the jumping motion and the reaction to warmth is an evolutionary device that has helped the seeds get out of the hot Mexican sun and into cooler damper places more suited for survival of the moth when it exits the hull. Indeed, on hot days the motion of the pods in the hot sun beneath the jumping bean shrub is significant enough that the sound can be heard for quite a distance and is said to resemble rain on the dry leaves.
The caterpillar lives inside the seed for several months. Right before it forms a cocoon for its final metamorphosis, the caterpillar cuts a circular hole in the hull of the seed. It then goes into a cocoon and changes to a moth, escaping the seed by pushing the lid off of the hole it created earlier.
More than sixty years ago, a twelve year old Mexican boy came up with a plan to market Mexican jumping beans. Senor Joaquin Hernandez, the legendary "king of the jumping beans started selling the beans that he harvested in the area of Alamos, Mexico." The industry was successful and today Hernandez pays local people to collect and bring the inhabited seeds to him. He employs over 50 people to sort and package the seeds, then ship them to sellers. Some years Hernandez exports over 20 million Mexican jumping beans.
Mexican jumping beans aren't alone in the active seeds category. There is a lesser known shrub Sapium biloculare that also produces jumping pods, and is called the Arizona jumping bean, although the plant's range extends well beyond Arizona. There are also "jumping galls" found in the Sacramento Valley of California where a wasp species forms a gall on the leaves of native oaks, then causes the tiny gall to hop about on the ground when the gall drops to the earth. There is also the Idaho leaping potato, but that's a whole different story, and one best not told here.