Take a matchbox
and find a victim
Ask the victim for a £20 note, and place it in the matchbox. At a £20 note of your own.
There is now £40 in the box. Close the box.
Ask the victim (who is preferably drunk): "would you give me £30 in exchange for that box and its contents?"
If the victim is dazzled enough by your patter, they accept -- after all, £40 for £30 is a £10 profit.
... or is it?