Patricia Evans, age 13, of Atlanta, Ga., for her question:

HOW DOES AN OIL WELL WORK?

Drilling an oil well takes great skill. A cable tool drilling crew sets up and operates special equipment. Usually there's a derrick constructed and then rotary¬drilling equipment powered by diesel engines sends a metal bit into the earth. It's an extremely complicated business with long sections of pipe added as the shaft goes deeper into the ground.

When the drilling bit strikes oil it is time to install a Christmas tree. This is a group of control valves placed at the upper end of a network of tubings and casings. Pipes lead from the Christmas tree to a tank that separates the oil from natural gas. Most oil wells stop flowing naturally after a short while, and then it's time to install a pump. A "walking" beam is installed which moves up and down as it sucks oil from the well. About 90 percent of the wells in the United States use such pumpers.