In a mathematics class the teacher told the student that minus * (times) minus –*– = + and also NO + NO = Yes. So, a boy was pressed to go to toilet. He then asked the teacher, "Excuse me, sir, can I use the toilet?" The teacher said, "No." The student asked again, "Excuse me, sir, can I use the toilet?" The teacher said, "NO."

Immediately student stood up to go to the toilet, the teacher was surprised and said, "Where are you going?" He said, "Toilet." The teacher asked him, "Why?" He said, "NO + NO = yes, so since you said NO two times I know you mean yes." So the class burst into laughter.