On Monday, Robert promises to put new siding on my house for $5,000. On Tuesday morning, I accept by phone call and promise that I will pay him $5,000 upon completion of the job. On Tuesday afternoon, Robert revokes his offer, telling me that he's retiring from the siding business. I respond saying: "Dude, we have a contract. Either put on the siding for $5,000 or I'll sue you." Explain why I am correct in this response.