A Promise v. an Offer: You need Consideration for a Contract

Pacta sunt servanda (agreements must be kept)

However, under United States common law, not all promises made in offers and acceptances are enforceable. The doctrine of consideration is the theory that separates those promises in offers and acceptances that will be enforced by the courts from those that will not. If an agreement is not supported by valid consideration, that agreement is not an enforceable contract.However, certain promises in an otherwise unenforceable agreement may be enforced under a promissory estoppel theory.

How Consideration is Defined:
The definition of consideration varies. No one version is accepted as covering the breadth of the doctrine.

1. The Benefit/Detriment Theory
Under this theory, a promise is deemed supported by consideration (and this enforaceable), whenever:

(1) The promisee either acts, or promises to act, in exchange for the promisor's promise; and

(2) The promisee's act or promised act is either a legal detriment to the promisee or a legal benefit to the promisor.