If you haven't seen the following before you may enjoy thinking about it. It's something I remembered from way back.
Three friends eat in a restaurant. The bill is £30, so everyone puts in £10. However the manager realises the bill should only have been £25, so he asks the waiter to give back £5. Each diner puts a pound back in their pocket and they give the remaining £2 to the waiter as a tip.
The diners have now paid £9 each, which comes to 3 x £9 = £27. With the £2 tip that makes £29. Where has the remaining £1 gone?