Use a bottle opener to remove the metal cap from a bottle and empty the bottle of its contents by drinking it or pouring it into a glass.
Bend the bottle cap until it is completely folded in half.
Lay the empty bottle on its side on the bar counter or on a table.
Place the folded bottle cap just inside the mouth of the bottle so that more than half of the bottle cap is inside the bottle at the mouth and a little less than half is sticking outside of the bottle.
Place a bet with a friend that they cannot blow the bottle cap inside the bottle. This looks really easy to do; however, when your friend tries to blow into the bottle, the bottle cap will pop out of it every time because the air goes inside the bottle and behind the bottle cap, forcing it out of the bottle.
Collect your winnings from the bet after your friend gets frustrated and gives up trying.