Make Money By Magic Money Trick
Ask someone to lend you $20 to put in an empty matchbox, you then also put $20 in. (You must point out the fact 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.