How urgent?

Dear Doctor Science, Why does moss grow on the north side of the tree? Please give me a very detailed response. THIS IS VERY URGENT! -- Nathan Palmer from Seattle, Wash.

You Microsoft employees crack me up. So there you are on your day off, stumbling around lost on the Olympic Peninsula, your GPS batteries are dead and your expensive Gore Tex-lined hiking boots soaked. So whatta you do? You pull out your Palm Pilot and fire off a missive to drscience.com and demand an answer. Well I've got some bad news for you. Moss only grows on the north side of the tree in places that have sunlight more than one day a year. To mix a metaphor, that kind of puts you up the creek without a paddle, doesn't it? E mail Bill G. and see if he can send his private helicopter.