Anna Maria Cafarelli, age 11, of Somerville, Mass., for her question:

Where does a compass point at the equator?

The earth is a giant magnet with two opposite magnetic poles. A magnetic compass is a small magnet with two opposite magnetic polls. The north poles of two small magnets repel each other and spring apart. The north and south poles of two small magnets attract each other and tend to cling together. The two poles of a magnetic compass needle are attracted by the two magnetic poles of the giant earth.

One pole of the compass magnet points to the earth's north pole, and the opposite end points to the earth's south pole. The same end of the compass needle always points north and the opposite end always points south. The magnetic compass does not change as it crosses the equator.