Eric Snyder, age 10, of East Brunwick, New Jersey, for his question:

HOW DO YOU KEEP YOUR BALANCE?

Three fluid filled semicircular canals in the inner ear let us keep our balance when we sit, stand, walk, run or ride a bicycle. These canals    enable us to recognize the direction    in which our bodies are moving. Even with our eyes closed, we know if we are going forward or backward, up or down, or sideways.

One canal is horizontal while the other two are in a vertical position. When the head or body moves, the fluid in the canals moves, affecting delicate hair cells located at one end of each canal. These hair cells are supplied with fine fibers from the vestibular nerve, which carry impulses to the brain.

The brain immediately interprets the impulses as changes in the body's position. Then the brain sends messages to the various muscles that maintain the balance of the body.