A team of engineers were required to measure the height of a flag pole. They only had a measuring tape, and were getting quite frustrated trying to keep the tape along the pole. It kept falling down, etc.
A mathematician comes along, finds out their problem, and proceeds to remove the pole from the ground where he can measure it easily.
When he leaves, one engineer says to the other: "Just like a mathematician! We needed to know the height, and he gave us the length!"