A little fun you can have to demonstrate centrifugal force, but you might want to do it outside. Don: Ya?l, see this glass full of water? Ya?l: Yes. D: Watch me swing it around my head! Y: Look out!!!...Hey, how did you do that? You swung the glass completely around without spilling a drop. D: Not bad, eh? Want to try it? Y: Not until I know the secret. D: The secret is centrifugal. Y: You mean centrifugal force? I thought that was a myth. D: It's no myth, though it is what physicists call a "fictitious force." If you tie a rock to a string and spin it around, it feels as though some force is trying to drive the rock away from the center of the spin. What you're actually feeling, though, is just the rock's inertia. Y: Hmmm...better explain that. D: Well, the law of inertia says that something in motion will tend to stay in motion, and what's more, to move in a straight line. The rock is set into motion and is trying at every moment to leave the circle and move away in a straight line. The string is keeping it from doing so, which is why it feels like a mysterious force is pushing the rock away. Y: I get it. So when you spin the glass over your head, the water inside it is trying to move off in a straight line too. But the glass prevents it from doing so, as long as you keep the bottom of the glass pointed away from yourself. Inertia holds the water inside the spinning glass! D: Right. Let's try again. (SPLASH) Y: I think our listeners better practice this one outside at first.