Here's some BLL for you to chew on:
Meeting of the minds (objective standard for mistake by the parties, though don't get this confused with mutual mistake doctrine; Raffles)
Offer and Acceptance (mailbox rule; revoking under common law and Restatement 45; Brooklyn Bridge problem; difference between negotiation and offer)
Limited and Indefinite Promises (best effort situations like Lucy, Lady Duff Gordon; court dislike for indefinite contracts; Fuller)
Parol Evidence (integration rule, WWW Associates, PG&E)
Impossibility
Great. Now that you have a little taste of the BLL of Contracts, try to apply it to a fact pattern such as this:
Jimbo goes on vacation three times a year to Arbua. On his last vacation, he asked his friend, Duff, whether he would be willing to mow his lawn while Jimbo was away. Jimbo told Duff that he would happily let him have his lawnmower at the end, if he did a good job.
Being a Ks professor, Duff wanted to draft a contract for the work. The contract read: "I, Duff, hereby promise to mow Jimbo's lawn while he is away for valid consideration of $30." The contract made no specification of the recent parcel that Jimbo had acquired, which he had never cut. It also didn't say anything about the lawnmower. Jimbo actually had two lawnmowers, both of which were named "peerless." One of them was awesome and built Ford tough, while the other was a souped up shopping cart that couldn't mow much.
Before Duff could begin cutting the lawn, Jimbo was arrested on charges of homicide for driving recklessly while drunk (he was that excited about Aruba). Before the cops took him away, he yelled, "Jimbo, forget about mowing!!!" Duff didn't hear him and later tried to mow his lawn, but the rains were too heavy. He mowed it, anyway, and broke the lawnmower. When Jimbo returned, he wanted to sue Duff.
You are Jimbo's lawyer. Advise him on all liability, including damages. Be sure to specify the arguments available to both of the parties.