Terri Browne, age 12, of Tucson, Arizona, for her question:

What are interglacial periods?

The glacial refers to an ice age, and an interglacial period comes between one ice age and the next one, or the one before. It is, of course, a period of warmer global weather, though the summer temperatures may be only a few degrees higher than they are during an ice age. It may be just warm enough in the temperature zones to prevent the winter's snow from staying on the ground through the summer. When it begins, the glaciers begin to melt and recede. The warm spell of global weather may last many thousands of years. But if it is a true interglacial period, the climate eventually becomes cool enough for the next ice age glaciers to form.

The earth has suffered through four cruel ice ages during the past million years. The last one began its retreat about 20,000 years ago. Some earth scientists suspect that our present climate is an interglacial period. They cannot be sure because they cannot explain how ice ages are caused. So we do not know if or when another one can be expected.