Sandra Potter, age 13, of Grand Forks, N.D., for her question:

WHAT IS A SPHERE?

A sphere is a solid figure that resembles a ball or globe. The name "sphere" comes from a Greek word that means "ball." In geometry, mathematicians define a sphere as the location in space of all points a certain distance from a fixed point called the center.

The mathematician says that a sphere is a solid figure bounded ~by a single surface. The surface itself does not have any edges or :boundaries. Each point on the surface is the same distance from the center.

The radium of a sphere is the distance from the center to the surface. Or, the radius is any straight line drawn from the center to the surface.

The diameter of a sphere is twice the radius. Or, the diameter is any straight line, drawn through the center, whose ends stop at the surface.

A secant is any line that cuts through a sphere.

A chord is any line that joins two points on the sphere.

If a plane, or flat surface, passed through the center of a sphere, the plane produces a great circle of the sphere. The radius and diameter of a great circle are the same as the radius and diameter of the space. A great circle cuts a sphere in half. Each half of the sphere is called a hemisphere.