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　　Nicky and Mark are picnicking on a moonlight beach.

　　M: Look. The tide's coming in again.

　　N: I don't understand why we have high tides twice daily --- the moon only revolves around the earth once a day.

　　M: Well, it's because the earth and moon rotate around a common center of gravity that's located somewhere inside the earth.

　　N: Is that because the earth is more massive and so has greater gravitational pull?

　　M: That's right. Now, if you imagine a bowl of soup rotating around a point somewhere near its edge, what happens to the soup?

　　N: Centrifugal force pushes it toward the point furthest, away from the point of rotation.

　　M: Exactly! So the moon's gravity pulls the sea toward it on one side of the earth, creating one high tide.

　　N: Oh, and on the opposite side, the other high tide is created by centrifugal force.

　　M: Right. Just like the bowl of soup analogy. And we pass both high tides during each twenty-four-hour rotation of the earth on its axis.

　　N: Now I understand. You're so smart, Mark. Could gravity attract you to someone like me?

　　M: No. You're not big enough. Your mass is insignificant compared with the earth's.

　　N: Well, in that case, maybe I'll eat some more sandwiches!

　　---by Sean McCormack