This was originally posted on a horrible site called Myspace. When Myspace underwent a redesign in Fall 2010, hundreds of insightful reader comments that had been left over the years were lost. I have since deleted my account there.
In the evolution "debate" you frequently hear people discussing the definition of the word "theory" (as in the theory of evolution, germ theory of disease, etc.). That has been written about extensively, so I won't go into it here. But occasionally you also hear a creationist say, "If evolution is so true, why is it still a theory and not a law, like gravity?"
Here's the scoop: In science, a "law" is something that can be described by a simple mathematical equation, one that is never seen being deviated from in nature. So, yes, gravity is a law, because there's a simple equation that predicts how two bodies will react gravitationally to each other, given a couple of variables: their masses and their distances. And it always works out. Force equals mass times acceleration -- that's a law. Energy equals mass times the square of the speed of light is also a law. They're laws because you can test them by plugging in the numbers and comparing the real result to the predicted result, which are always the same.
Natural processes like evolution don't work like that. There's no equation to describe how evolution works, any more than there's an equation that describes how cells divide or how memory is stored in the brain. Biological systems -- unlike, say, the planets and the sun -- are complex and chaotic, with tens or hundreds of variables. You can predict exactly when a solar eclipse will occur because the bodies of the solar system are simple and obey Kepler's and Newton's laws of motion. But you cannot predict the weather three days from now with any certainty, because there is no "law of weather." There are too many variables; each particle of air is doing its own thing.
However, in meteorology we have non-law theories which are very useful and predictive. A cold front moving into warm, moist air will cause precipitation and maybe thunderstorms; that is the prediction from meteorological theory, and from observation we know it is very much a truth. The germ theory of disease predicts that if people inhale anthrax spores, they will contract anthrax. And we know that they will -- we know because the germ theory has been supported to the point of virtual certainty. Even if there isn't a simple equation expressing this knowledge.
"Why isn't evolution a law?" is just another fallacy that creationists and IDers throw up because it sounds half-reasonable, and because they don't know better. Don't let it flummox you.