Tom Steele, age 13, of North Vancouver, B.C., Canada, for his question:

Why do the distant mountains look blue?

Faraway objects shrink with distance, and on the horizon a range of lofty mountains may look like a row of small molehills. On the Earth, distant objects also lose their colors or at any rate often change their colors. Those distant mountains may look gray or bluish gray, and as we approach we find that their true colors are brown or green. This color change is caused by the Earth's atmosphere.

The air around us is a mixture of gases, and gases are made from separate atoms and molecules. There are countless trillions of these particles in a room, but there are not enough of them to change our view of the walls. However, gazing out across miles of open country, the number of air particles between us and the horizon is past the imagination. Also present in the air are fragments of soot and grime, dust and dirt. The air and this floating debris tend to change the colors of the distant mountains.