Harry saw Joe in his classic Buick at a local drive-in restaurant and talked about wanting to buy the car. Joe offered to sell it to him for $1500. Harry said he needed some time to think about it and that he would send Joe a letter. The next day, Joe changed his mind and mailed Harry a letter of revocation. The day after that, before Harry had received the letter of revocation, he sent a letter of acceptance to Joe. The next day, the two letters arrived at their respective destinations, Joe receiving Harry's letter of acceptance just after Harry received Joe's letter of revocation. Explain the legal liabilities of the parties.