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 display his humor, walked up and erased the "c" in "class." Noticing the laughter, Professor Tom turned around, walked back, looked at the student and then at the altered notice. Calmly, he walked up, erased the "l" in "lass," glanced at the flabbergasted student, and proceeded on his way.