Not every type of offer is revocable. One type of offer that cannot be revoked is the option contract (the promisor explicitly agrees for consideration to limit his right to revoke). Arlene tells Neil that she cannot make up her mind in ten days but that she will pay him $25 to hold the offer open for thirty days. Neil agrees. Arlene has an option to buy the car for $5,000; if Neil should sell it to someone else during the thirty days, he will have breached the contract with Arlene. Note that the transactions involving Neil and Arlene consist of two different contracts. One is the promise of a thirty-day option for the promise of $25. It is this contract that makes the option binding and is independent of the original offer to sell the car for $5,000. The offer can be accepted and made part of an independent contract during the option period.
Partial performance of a unilateral contract creates an option. Although the option is not stated explicitly, it is recognized by law in the interests of justice. Otherwise, an offeror could induce the offeree to go to expense and trouble without ever being liable to fulfill his or her part of the bargain. Before the offeree begins to carry out the contract, the offeror is free to revoke the offer. But once performance begins, the law implies an option, allowing the offeree to complete performance according to the terms of the offer. If, after a reasonable time, the offeree does not fulfill the terms of the offer, then it may be revoked.