Professor Tom was to meet his students the next day, so he wrote on the blackboard: "Professor Tom will meet the class tomorrow."
A student, seeing a chance to show his humor, walked up and erased the "c" in "class." Noticing the laughter, the professor turned around, walked back, and looked at the student and then at the altered notice. He calmly walked up, erased the "l" in "lass," glanced at the flabbergasted student, and went on his way.