An offer is revoked when the offeror "takes it back" before the offeree accepts. In general, the offeror may revoke the offer any time before it has been accepted. Imagine that I call you and say, "I'm going out of town this weekend. I'll sell you my ticket to this weekend's football game for $75." You tell me that you'll think it over and call me back. An hour later, my plans change. I call you a second time and say, "Sorry, but the deal's off—I'm going to the game after all." I have revoked my offer, and you can no longer accept it.