Class, Lass and Ass
 Professor Tom was going to meet his students on the next day,
so he wrote some words on the blackboard which read as follows: Professor Tom will meet the class tomorrow.
A student, seeing his chance to display his sense of humor after reading the notice, walked up and erased the c in the word class. The Professor noticing the laughter, wheeled around, walked back, looked at the student, then at the notice with the c erased--calmly walked up and erased the l in lass, looked at the flabbergasted student and proceeded on his way.