Beer Bottle Cap Trick
Challenge:
Take an ordinary beer bottle cap fold it in half so it forms a taco shape.
Place the folded bottle cap in the neck of a beer bottle and challenge your customer to blow the cap into the bottle.
They will think its easy but probably find it impossible as the cap keeps jumping out of the neck ???
You then try the challenge and with a cheeky method, succeed first time, winning the challenge / bet.