Mary Kathryn Bayer, Age 9, Of Rochester, N.Y.., for her question:

What causes gravity?

We cannot see it or hear it or smell it, but the force of gravity is all around us. It is a built in part of the earth, the moon and all the stars. The force of gravity is even in our bodies. Every speck of dust has had its share of gravity since the universe was born billions of years in the past. But until almost 300 years ago, nobody noticed it.

We live with the force of gravity day and night every week of the year. It hugs us to the face of the globe and keeps our bodies from floating off into space. It pulls down the ripe apples from the trees and makes a heavy stone sink through water. It is gravity that gives weight to an object and makes your body heavy or light.

The force of gravity we feel is a built in part of our planet, the earth. A stone has a sma11er share and a grain of sand has a still smaller share of gravity. For this strange force is a built in part of every object from the giant stars to the tiny specks of dust. We take it for granted that heat is part of a blazing fire. We must also take it for granted that the force of gravity is a part of objects made of matter.

People learned long ago that bulky objects are heavy, that if you stumble, you tumble dawn, and that a stone falls down to the ground when you drop it. From the very beginning of the human family until. About 300 years ago, people just took the force of gravity for granted. Then a young student named Isaac Newton began to wonder why a falling object always went down to the ground and not up or in any other direction.

Newton figured that there must be an invisible pulling force within the earth. It also must be present, he figured, in every body of matter. He studied the movements of the moon and the planets, and he reasoned that the force of gravitation reaches out and makes all solid bodies attract each other. He figured out that the strange force follows certain rules, and, in the year 1687, he wrote down the laws of gravitation. But he did not explain what causes the gravity which is built in to all bodies of matter. And nor can we.

The earth's gravity is stronger than the moon's. This is because the earth has more material or matter. The gravity of the earth and the moon reach out, and the two round bodies attract each other. But gravity grows weaker as it goes. The earth's gravity cannot pull down the distant moon like a falling stone.

The force of gravity depends upon the amount of matter in an object. In a grain of sand, it is too small to measure. In the massive sun, the force of gravity is strong enough to keep the planets in their places. And all of them arc millions of miles away, with the sun's gravity getting weaker and weaker all the way.