In general when we refer to a theory we mean something that's not proven yet. In science it's different, as Yael and Don discuss in this Moment of Science.   Y: Today, on "A Moment of Science," we clear up the difference between a scientific theory and a scientific law.

D: Let me guess. You're talking about that argument that if the theory of evolution were true, it would actually be a law.

Y: Yes--I'm tired of hearing people argue that the fact that evolution is a theory means that modern science itself isn't convinced it really happens.

D: Yeah, me too.

Y: And that's why I thought it's worth going over the terminology one more time.

D: Well, the definition of a law is easy. It's a description--usually mathematical--of some aspect of the natural world.

Y: Like gravity.

D: Exactly. The law of gravity describes and quantifies the attraction between two objects. But the law of gravity doesn't explain what gravity is or why it might work in this way. That's because that kind of explanation falls into the realm of theory. And the theory that explains gravity is the theory of general relativity.

Y: Right. According to the National Academy of Sciences, a scientific theory is a "well- substantiated explanation of some aspect of the natural world that can incorporate facts, laws, inferences, and tested hypotheses." In other words, all scientific theories are supported by evidence, and you can test them, and--most importantly--you can use them to make predictions.

D: So based on that definition, theories never change into laws, no matter how much evidence out there supports them. Formulating theories, in fact, is the end goal of science.

Y: So to say evolution is just a theory is actually an argument for it and not against it. You can't do any better in science than to be a theory.