He invited the devil to have a drink with him. Jack did not want to pay for the drink. He persuaded the devil to change himself into a coin. The devil did so, and Jack put the coin in his pocket. There was a silver cross in his pocket that made it impossible for the devil to return to his original shape. Jack eventually allowed the devil to resume his shape, but only after he promised not to bother Jack for a whole year and to not try to claim his soul when he died.