Its coming to the end of the school year here so the kids are enjoying some slack in lessons. Anyway, they were given this little logic task, with the first correct answer winning a small prize.
According to the source, Albert Einstein reckoned that 98% of folk couldn't complete it. I managed it after a few fudged attempts with scraps of paper and coloured pens, but I switched to Excel and found it easier to move things around, and then things snapped in to place quite quickly.
If anyone wants to test themselves, here's the puzzle: http://www.mathsisfun.com/logicpuzzle.html