Annalisa Salonius, age 11, of Fredericton, N.B., Can., for her question.

WHY DID THE ICE AGE GLACIERS RETREAT?

Several times in the past million years, mighty glaciers advanced over much of our continent and then retreated. These ice ages happened because the climate became colder. Winters lasted longer and summers were too short and too cool to melt all the snow.  In time, the climate became warmer again  warm enough to melt the ice and cause the great glaciers to retreat. Nobody is sure what caused the changes in the climate.