I like this:
"Science has proof without any certainty. Creationists have certainty without any proof." - Ashley Montague
Science has never claimed to know it all, or that whatever it knows is unchangeable. Science is dynamic, as it constantly gathers new information to fit into the puzzle. Hypotheses are always being tested again and again to make sure that the results are correct and consistent before they can be called theories. Creationists, on the other hand, say they know how the earth came to be, denying the scientific evidence that proves otherwise, that has been tested and peer-reviewed and published many times.
"MY book says it's so, therefore it's true." That's not good enough. Maybe a few thousand years ago it was, but not anymore.