What are the requirements for performance to satisfy the Statute of Frauds in a service contract?

Hypo 1: Bob tells Sam that he will give him his speed boat if Sam agrees to mow his lawn every week for 3 years. After 3 years of mowing Bob's lawn, Sam knocks on Bob's door and says, "I finished! Now give me your boat." Bob slams the door on Sam's face. Result: People generally don't mow other people's' lawns for free. The fact that Sam mowed Bob's lawn every week for three years is evidence that there was likely some sort of reason or relationship. In fact, if Bob didn't actually agree to give Sam anything, then the reasonable thing to do would have been to have a talk with Sam and ask him why he was mowing the lawn every week. Instead, Bob sat back and enjoyed the benefit of Sam's work. Given these facts, a court will allow Sam's full performance of the agreement act as satisfaction for the Statute of Frauds.

Hypo 2: Bob and Sam are having coffee. Bob says, "Man, I'd give anything to not have to mow my own lawn for 2 years." Sam responds, "Really? Would you give your speed boat?" Bob laughs and says, "Yeah, sure." Sam says, "Agreed!" The next day Bob finds Sam mowing his lawn. Bob says, "Woah, what are you doing?" Sam says, "I'm mowing your lawn for your speed boat." Bob says, "I was just kidding -- please stop." Result: Sam's partial performance of the agreement is not sufficient to satisfy the Statute of Frauds. However, if Bob had said nothing and allowed Sam to continue mowing his lawn for 2 years, Sam's performance would be sufficient to satisfy the Statute of Frauds.