Mike Kendall, age 13, of Carson City, Nev., for his question:

WHAT IS HEAVY WATER?

Heavy water is water that contains the heavy isotope of hydrogen called deuterium. The deuterium atom weighs about twice as much as an ordinary hydrogen atom.

Heavy water, which is also called deuterium oxide, makes up about one part in 4,000 to 6,000 of ordinary water.

Heavy water freezes at 38.88 degrees Fahrenheit, rather than at 32 degrees. It boils at 214.56 degrees Fahrenheit rather than at 212 degrees.

Heavy water is useful in some kinds of nuclear reactors. It acts as a moderator, to control the energy of the neutrons in a chain reaction. It may also act as a coolant, removing the heat produced in the reactor core.