Ask someone to lend you £20 to put in an empty matchbox, you then also put £20 in. (You must make this obvious that you have also put in £20) Say to them that you will sell them the matchbox for £30. Believing that they are in for a bargain (spending £30 to buy a £40 matchbox). But the trick is that because they already have put in £20 they are in fact losing £10. (In total they have spent £50 for a £40 matchbox, and you get £30 instead of the £20 you started with. It is now a good time to congratulate them on their win and make a fast exit before they have time to work it out.