The owner of a stationary bicycle wrote a letter to her friend offering to sell her stationary bicycle to him for $150. The friend received the letter on January 18. On January 19, he mailed a letter back saying that he was not interested in purchasing the bike because he had just purchased a gym membership. However, the friend changed his mind the next day and mailed a letter to the owner accepting her offer to sell the bicycle and enclosing a certified check for $150. The owner received the friend's rejection letter on January 21 but put it aside without reading it. The next day, she received the friend's acceptance letter, which she opened and read immediately. Do the parties have a contract?