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The Professor and the Plumber

One professor of mathematics noticed that his kitchen sink at his home broke
down. He called a plumber. The plumber came on the next day, sealed a few
screws and everything was working as before. The professor was delighted .
However, when the plumber gave him the bill a minute later, he was shocked.

"This is one third of my monthly salary!" he yelled. Well, all the same he
paid it and then the plumber said to him: "I understand your position as a
professor. Why don't you come to our company and apply for a plumber
position? You will earn three times as much as a professor. But remember,
when you apply, tell them that you completed only seven elementary classes.
They don't like educated people."

So it happened. The professor got a plumber job and his life significantly
improved. He just had to seal a screw or two occasionally, and his salary
went up significantly.

One day, the board of the plumbing company decided that every plumber has to
go to evening classes to complete the eight grade. So, our professor had to
go there too. It just happened that the first class was math. The evening
teacher, to check students' knowledge, asked for a formula for the area of
the circle. The person asked was the professor.

He jumped to the board, and then he realized that he had forgotten the
formula. He started to reason it, he filled the white board with integrals,
differentials and other advanced formulas to conclude the result he forgot.
As a result he got "minus pi times r square".

He didn't like the minus, so he started all over again. He got the minus
again. No matter how many times he tried, he always got a minus.

He was frustrated. He looked a bit scared at the class and saw all the
plumbers whisper:

"Switch the limits of the integral!!"