Lisa Marie Horner, age 9, of Sioux City, Iowa, for her question:

Which is the smallest bone in the human body?

Perhaps you thought that your smallest bones are the bones at the ends of our fingers. Not at all. There are six much smaller bones buried deep inside your head. They belong to the hidden parts of your ears. These ear parts that are buried deep inside your skull do the real hearing work for you. Those shell like ears on the sides of your head just funnel in the sounds. There are two hidden working parts for each ear. One is the middle ear and the other is the inner ear. They are both made of many delicate working parts. But the middle and inner ear together are not much bigger than the little joint of your finger.

In each middle ear, there are three tiny bones that tap out the pulsing vibra  tions of sounds. One is a pixie sized hammer fixed to a tiny drum about 1/4 inch wide. It is linked to two other moveable bones called the "anvil" and the "stirrup." Sounds make the tiny hammer bone shiver and tap out a message on the ear drum. It works with a tiny anvil bone and a stirrup bone to send sound messages to the brain. The anvil bone is a smidgen smaller    than the hammer bone.