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 and measure it easily.
When he leaves, one engineer says to the other: “Just like a mathematician! We need to know the height, and he gives us the length!”
This joke is included in the Black Book of Jokes. You can order it from here.