In a unilateral contract, acceptance happens by performing the requested act, not by making a promise. The offer promises payment in exchange for completing a specified task, so the offeree accepts by starting that task. Once the offeree begins performance, the contract is formed and the offeror is obligated to pay once the act is completed (or as specified). Merely promising to perform later isn’t acceptance, and asking questions about terms shows no acceptance yet. A written contract isn’t required for formation in a unilateral contract. For example, if someone offers to pay you to paint a fence, starting to paint constitutes acceptance and forms the contract, even if you haven’t finished yet.

Promissory estoppel is the principle that allows enforcing a promise without new consideration when the promise is relied on to the promisee’s detriment to prevent injustice. When someone makes a clear, definite promise and the other person reasonably relies on it, acting or forbear­ing in reliance (such as spending money or giving up a benefit), the law can enforce the promise even though there isn’t a new bargain supported by consideration. The key elements are a definite promise, reasonable reliance by the promisee, actual detriment from that reliance, and the need to prevent injustice. The parol evidence rule, statute of limitations, and the mailbox rule address different legal issues—interpretation of written contracts, timing of when a lawsuit must be filed, and when an acceptance becomes effective, respectively—so they don’t govern enforcement based on reliance without consideration.

The main idea is that a life-contingent contract allocates the risk of the payee’s death to the person who buys the arrangement. A life annuity is payable for the life of the annuitant, so the contract ends when that person dies. By purchasing an annuity to be paid for her life, the ex-husband assumed the risk that she might die before any payments are made. When she dies, there is no obligation left to perform, and there’s no basis for rescission because there was no misrepresentation or other improper conduct. So he cannot rescind the contract. The other options don’t fit: death of the payee does not “nullify” the contract in a way that requires rescission; the fact that the annuity is for life explains why payments wouldn’t occur after death rather than providing a right to rescind; and there’s no misrepresentation shown.

For a contract offer to be valid under common law, there must be a definite and certain manifestation of willingness to enter into a contract, communicated to the offeree, with the intent to be bound upon acceptance. Definite and certain terms ensure the terms are knowable and enforceable if the offer is accepted. The offer must be communicated to the person who could accept, so they have knowledge of the terms and the ability to assent. And there must be an intent to be bound simply upon acceptance, not merely a preliminary invitation or wish. An invitation to negotiate lacks the definiteness of a contract offer, so it isn’t itself an offer. A counteroffer operates as a rejection of the original offer and creates a new offer, not a valid offer standing on the original terms. An improper motive to cause harm isn’t what makes an offer valid; it doesn’t address whether the terms, communication, and intent meet the requirements for an offer.