One Hand Clapping

On Wednesday, a student asked the old master, "What is the sound of one hand clapping?" The master paused a moment and then with his open hand struck the student sharply across the face. "What did you do that for?" asked the stunned student. "I don't know anything about one hand clapping, but that was the sound of one hand slapping!" replied the old master. "Why do you ask me such a ridiculous question?"