The story is told of an atheist philosophy professor who performed a parlor trick each term to convince his students that there is no God. "Anyone who believes in God is a fool, " he said. "If God existed, he could stop this piece of chalk from hitting the ground and breaking. Such a simple task to prove he is God, and yet he can't do it." The professor then dropped the chalk and watched it shatter dramatically on the classroom floor.
If you meet anyone who tries this silly trick, take the roof off. Apply the professor's logic in a test of your own existence. Tell the onlookers you will prove you don't exist.
Have someone take a piece of chalk and hold it above your outstreatched palm. Explain that if you really exist, you would be able to accomplish the simple task of catching the chalk. When he drops the chalk, let it fall to the ground and shatter. Then announce, "I guess this proves I do not exist. If you believe in me, you're a fool."
Clearly, this chalk trick tells you nothing about God. The only thing it is capable of showing is that if God does exist, he is not a circus animal who can be teased into jumping through hoops to appease the whim of foolish people.- Greg Koukl, Tactics: A Game Plan for Discussing Your Christian Convictions, p.150, 151