Barry Sanders, age 10, of Columbia, Tenn., for his question:

What caused the La Brea tar pits?

The vacation season is with us, and many easterners and midwesterners will be visiting our western mountains and Pacific shores. Some of those who visit Los Angeles will go to see the amazing La Brea tar pits. These gooey puddles are made of tar and asphalt seeping up from below the ground. They are possible because this region sits upon vast reservoirs of buried petroleum.

Some of this petroleum at the tar pits seeps to the surface and spreads onto the ground. The petroleum contains quantities of volatile gases, light enough to rise up and go off into the air. The heavier, tacky substances once blended with the light gases of the petroleum are left behind. They form the tacky tar pits of the La Brea ranch.