Professor 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, he paid 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 forgot 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!"