Some engineers are trying to measure the height of a flag pole.
They only have a measuring tape and are quite frustrated trying to keep the tape along the pole.
It falls down all the time.
A mathematician comes along and asks what they are doing.
They explain it to him.
"Well, that's easy..."
He pulls the pole out of the ground, lays it down, and measures it easily.
After he has left, one of the engineers says: "That's so typical of these mathematicians! What we need is the height - and he gives us the length!"