Economics
The Elements of a Valid Contract: Offer, Acceptance, and Consideration
Quick fact
A contract is not valid unless it includes a bargained-for exchange of something of value (consideration) between the parties; a promise to make a gift is not enforceable because the recipient gives nothing in return.
Why this is interesting
You probably make dozens of promises every week—but what turns a simple 'I'll do it' into a legally binding agreement that a court will enforce?