Exchange of value which the law will enforce. Purpose of contract is to promote trade and commerce. Do not have to be signed to enforce. Offer acceptance is taken place when you pay. Rejected(if some wants to buy something from you at a price, you say no but after you accept it. This is not an offer because it was rejected) Counter-offer(a rejection to a offer and also a new offer) Revoked(i can revoke my offer anytime before they accept it) Expire(can not accept an offer after it expire) the period before it expire are reasonable, can be different. One of the party dies or become incompetent(the death of the party will not terminate the contract) I say i will make my offer open however i change my mind, this is not a contract because there is lack of consideration. When acceptance may be made by mail, the acceptance is effective when mailed, not when received.