I always thought these were pure magic! No one bothered to explain anything about "ball bearings". I remember being a bit freaked out.

Mexican jumping beans (also known as Frijoles saltarines), native to Mexico, are seed pods that have been inhabited by the larva of a small moth, (Cydia deshaisiana). The bean, usually tan to brown in color, "jumps" when heated, because the larva spasms in an attempt to roll to a cooler environment. The shrub that yields these beans is often also referred to as the jumping bean. The heat from the sun stimulates the larvae to "jump" to avoid dehydration and consequent death, even in relatively cool environmental temperatures. After the egg hatches, the larva eats away the inside of the bean (until it becomes hollow) and attaches itself to the inside of the bean with silk-like thread. The larva may live for months inside the bean with varying periods of dormancy. If the larva has adequate conditions of moisture and temperature, it will live long enough to go into a pupal stage. In the spring, the moth forces itself out of the bean through a round "trap door", leaving behind the pupal casing. After its metamorphosis, the small, silver and gray-colored moth lives for no more than a few days.

Liuttle kids would swollow these