Consideration is the central concept in the common law of contracts and is required, in most cases, for a contract to be enforceable. Consideration is the price one pays for another's promise. It can take a number of forms: money, property, a promise, the doing of an act, or even refraining from doing an act. In broad terms, if one agrees to do something he was not otherwise legally obligated to do, it may be said that he has given consideration. For example, Jack agrees to sell his car to Jill for $100. Jill's payment of $100 (or her promise to do so) is the consideration for Jack's promise to give Jill the car, and Jack's promise to give Jill the car is consideration for Jill's payment of $100.