Gozer is a traveling salesperson for ECTO sporting goods. While having a cocktail at the hotel bar, Egon, the athletic director of the local high school, sits down next to him, orders a scotch, and the two men start talking. After three hours of talking and drinking together, Gozer scribbles on a bar napkin that he will sell $100$ volleyballs to Egon for a total of $$500.$ Both men sign the napkin and as they get up from their seats to shake hands, Egon stumbles forward. Which element of a contract is at issue in this situation?