This tutorial concludes by considering one more way in which an offer may become irrevocable: by beginning performance.
Consider this example. Alice offers Bob $500 if he will walk across the Michigan Avenue bridge. In response, Bob begins to walk across the bridge. When he is halfway across, Alice shouts, "I hereby revoke my offer."
Should Alice be allowed to revoke her offer? It certainly seems unjust to allow her to do, and that is indeed the law's view.
The law distinguishes two situations here: (1) those in which the offer invites acceptance either by a performance or by a promise; and (2) those in which the offer invites acceptance only by a performance and not by a promise. (Offer and Acceptance Tutorial 3 discusses the ways in which an offer invites acceptance.)