Yale Law professor Ian Ayres walks through Carlill v. Carbolic Smoke Ball, the classic English contract case about a company that advertised a cash reward to anyone who used its smoke ball product and still caught influenza. Ayres uses the case to teach how courts determine when a unilateral offer becomes binding and what counts as valid acceptance through performance rather than a returned promise. The lecture is part of his American Contract Law course, originally built for Coursera, covering the bargain relationship and the mechanics of exercising a power of acceptance. He works through the court's reasoning on the company's deposit of money as evidence of serious intent, and on the plaintiff's use of the smoke ball as the act that closed the deal, using the case to sharpen students' sense of when an advertisement crosses the line into an enforceable offer.