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?

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