Kathy Fox, age 11, of Niagara Falls, Ontario, Canada, for her question:

Is the eye iris useful?

The iris is that circular band of color around the black pupil in the center of the eye. It may be blue or grey, brown or speckled hazel, but whatever its color, the iris is bright and beautiful. However, it has more to do than just look pretty. If it failed to perform its duties, the eye would be unable to cope with the light rays that carry pictures to its viewing screen. The iris is shaped like a perfect doughnut. The hole in the middle is the pupil, the window that lets in the light.

In order to see, the eye needs to receive enough but not too much light. The iris contracts and expands to adjust the right amount. In brilliant sunshine it becomes larger and the pupil becomes smaller. This reduces the amount of incoming light. In the evening, the iris shrinks and the pupil window becomes bigger to gather as much of the dim light as possible. The iris has two sets of strong sensitive muscles that work automatically to make the pupil larger or smaller.