A very old problem concerns a tall building and three experts, an physicist, a mathematician, and an engineer.

Each was given a watch, a long piece of string, and a level, and was asked to determine the height of the building.

The mathematician tied the watch to the string, went to the top, and swung it like a pendulum, and calculated the length of the string from the frequency of the pendulum. His answer was off by about 10 per cent.

The physicist went to the top, read the time on the clock, and then dropped it. Comparing the time elapsed to when the clock stopped (literally) he calculated the free-fall time to deduce the height, and his answer was off by about 7 per cent.

The engineer went inside, and reappeared with the exactly correct answer.

How did he do it?

(Note: it's a bit of a trick question?)

John