Reminds me of the time I solved this problem:

A man walks towards his house at 4 miles an hour. At the time he sets off, a fly leaves the man's house, flying at 8 miles an hour. When it reaches the man, it turns around and flies back to the house. It keeps repeating this journey until the man reaches the house an hour later. How far did the fly fly?

I spent ages working out where their paths intersected and how far the fly flew each circuit, ending up with an infinite series. On summing the series, I slapped my head and let out the oh so classic exclamation of "d'oh".

cLive ;-)

PS, for those of you as clueless as me, highlight below:

The fly flew 8 miles, since the man walked for one hour and the fly flew for one hour.