Robert Sebesta, age 14, of Crosby, Texas, for his question:

Which is the thickest layer of the atmosphere?

There is enough air above our heads to supply everyone in the world with about two million tons of it. You can write the weight of the entire atmosphere with 5, followed by 15 zeros, and half of this tremendous weight is contained in a layer 3.6 miles deep near the surface of the earth. The layer above it, another 3.6 miles in depth, contains half the remaining weight of the air so that three quarters of the weight of the atmosphere is contained in a layer about seven miles deep at the surface of the earth. The thickness of the air. is, of course, its density ‑ the number of gas molecules packed into a given volume. The air near the surface is under the terrific weight of the entire atmosphere above it. This weight is about one ton over every square foot at sea level. The air becomes less dense the higher we go until, hundreds of miles above our heads, it finally peters out in empty space.