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.