Larry Robinson, age 9, of Portland, Maine, for his question:

Why doesn't Mercury turn around like the other planets?

Let's begin with the moon, which is nearer to home. The face it shows to us is always the same face. The same dark patches show up in the same places and the golden moon swells and shrinks. It never shows us the back of its head. Since we can see only one side of the moon, you might think that the big ball never turns around as the earth does, from day to night. But it does. It spins around slowly, once every time it makes a trip around its orbit. It turns so slowly because the pulling gravity of the earth stops it from turn ing faster.

The little planet Mercury is very close to the mighty sun. And the sun's mighty gravity is much stronger than the earth's. It pulls at little Mercury more than 20 times harder than the earth pulls at the moon. The sun does not make Mercury stop turning around. But its mighty pull refuses to let the little planet spin around fast. It slows Mercury down, just as the earth slows down the turning moon. In fact, like the moon, Mercury spins so slowly that one side never gets a chance to face the sun.