The contract must create future obligations. "A" gives a deed of his house to "B." There is no contract; the transaction is complete. The theory and intent of a contract is to create a binding obligation upon each of the parties to do or abstain from doing something in the future. If the thing is done, it would be nonsense to attempt to contract with reference to it. Naturally, also, the offer and acceptance must relate to a specific thing, known to both parties. No contract is created if each has a different thing in mind. There is no meeting of the minds under such circumstances. A mutual mistake will avoid the agreement.