This article proposes that a history-of-knowledge approach offers innovative ways to study the use of domestic infrastructure in the household. More specifically, the article investigates the role of knowledge about water fixtures, such as meters, taps, and toilets, in the history of progressive-era Los Angeles. Building on the rich literature about how Los Angeles obtained its water, this article shifts the focus to the relationship that everyday consumers had with their water and how technology mediated this relationship. While the article analyzes three major fields of knowledge about the use of infrastructure (knowledge about personal and public hygiene, about the maintenance and repair of fittings, and about responsible levels of water consumption), it foregrounds users’ agency in construing bodies of knowledge. Taken together, this article argues, first, that practical knowledge about water as a modern convenience was mutually developed by the utility’s publicity department, meter men, municipal health authorities, elected officials, newspaper editors, middle-class reformers, property owners, working-class immigrants, and female householders. Second, the article emphasizes the dynamics, contingency, and locality of this knowledge, which was linked to the stunning growth of Los Angeles between 1880 and 1930.