The promise of the one party to the contract must be supported by an undertaking of the other. Each must obligate himself. Each must put some consideration into the agreement. A mere promise would not be binding upon its maker. A, seeing his good friend B, says to him, "B, I will give you my house to-morrow." B cannot enforce the delivery of the house. But if A had made offer to give B the house if B would cease the use of tobacco for one week, then there is a mutual obligation or consideration and B, having performed his promise, can enforce delivery of the house to him. In real estate contracts the usual situation is that the seller promises to sell and convey realty and the purchaser accepts the offer and creates the mutual obligation by agreeing to pay a certain price for the realty. A more extended discussion of the various kinds and adequacy of consideration will be found in the chapter on deeds.