Ask Marilyn

We're always hearing that the supply of water is dwindling. But how can this be? There's nowhere but planet Earth (including the atmosphere) for the water to go. —Rudy James, Chicago, Ill.

Yes, the amount of water on Earth is constant. But only 2.5 percent of it is freshwater. Of that freshwater, only a tiny fraction is readily available. Almost all of it is frozen or in the ground. Rivers, an extremely important source of water for humans, contain barely .0002 percent of Earth's freshwater. So as the population explodes, human use of water expands, and pollution reduces the already limited amount available, the remaining volume of freshwater accessible to supply the demand grows smaller.

Four golfers want to play as twoperson teams. One suggests taking a ball from each player (they use different balls) and choosing two randomly. Their owners will be partners. Another person says they need balls from only three players. Is this correct? —Mitch Levin, Winter Park, Fla.

Yes. The latter is the same as setting one ball aside and choosing one of the other three to designate its partner. The set-aside ball has an equal chance of pairing with any of the others, and the result determines both teams of golfers. All three methods are equally random.