Here's a good one, it really had me stumped for a while. I first came across it when I was about 12 and it blew my mind.
Three people enjoy a meal at a restaurant. The waiter brings the bill for £30 so each person pays £10.
Later the chef realises that the bill should have only been £25 so she sends the waiter back to the table with £5. The waiter was not very good at Maths and could not figure out how to divide the £5 so he gave each person a £1 and kept £2 for himself.
So....the three people have paid £9 each for the meal.
£9 x 3 = £27
The waiter kept £2
£27 + £2 = £29
What happened to the other pound?