The Mexican Jumping bean is a tiny bean that has a moth larva in the center of it that controls the movement of the bean. You can purchase them in tiny clear plastic boxes and they will live for months when properly cared for. They were a fad from the fifties that is coming back into fashion again.
The moth larva inside the bean lives a quiet life in the dark. It creates a tiny atmosphere of webs that is lives comfortably in until it gets too hot inside the bean. When it gets too hot the larva will move its body making the bean jump or hop. You can set the beans in the sun or just hold them in your hand and enough heat will get into the bean and create the hopping action. Never leave your beans in the sun unattended since if they are in the sun too long they will dehydrate and die.
The larva will stay alive for months inside the bean. You can keep the larva alive longer by placing them in a small puddle of water that only covers the bean a quarter of the way. Never emerge your beans completely in water or you could drown your larva and your beans will no longer hop.
When I was a kid I kept mine in their box all the time and never let them come out. Maybe I was afraid they would run away or escape. I purchased some for my son and he takes them out of the box a lot and they have never escaped or run away on him.
It has been fun for me to see this blast from the past and my son can enjoy them again now. I found them at a little out of the way toy store for three dollars but you can buy them on line at http://www.beansthatjump.com/.
You can learn more about the bean and how to care for them at http://en.wikipedia.org/wiki/Mexican_jumping_bean.