Almost every contract an ordinary person enters is now formed by clicking a button, and the law of whether that click bound them is both settled and widely misunderstood. This article explains what actually makes an online agreement enforceable, and it is not the signature — it is notice and manifestation of assent. It traces the doctrine from Specht v. Netscape through Nguyen v. Barnes and Noble, Sgouros v. TransUnion, and Meyer v. Uber, and extracts the design rules courts actually apply. It explains what ESIGN and UETA do, which is far less than people assume: they remove the writing and signature obstacle and change nothing else about formation. It covers attribution, the audit trail, and the records that make an electronic agreement provable years later. It covers the unilateral modification clause and why a change-of-terms provision without notice frequently fails. It covers dark patterns, negative option billing, and the disclosure rules that now govern subscriptions. A fictional dispute runs throughout.