A college professor had the mysterious habit of walking into the lecture hall each morning and removing a tennis ball from his jacket pocket. He would put it on the corner of the podium. After giving the lecture for the day, he would once again pick up the tennis ball, place it into his jacket pocket, and leave the room. No one understood why he did this.
Then one day, a student fell asleep during the lecture. The professor never missed a word of his lecture while he walked over to the podium, picked up the tennis ball and threw it, hitting the sleeping student squarely on the top of the head.
The next day, the professor walked into the room, reached into his jacket, and removed a baseball. No one ever fell asleep in his class the rest of the semester!