Over the last thirty years, the way people drink water in the United States has changed dramatically. Tap water, once the de facto drinking water source for nearly all U.S. residents, is now the primary source for only one-third of the population – with bottled water and home-filtered water representing the lion’s share of the remainder. Simultaneously, governments at all levels are pouring billions of dollars into maintenance, refurbishment, and promotion of local water systems.This disjuncture – a phenomenon I call the “tap gap” – is poorly understood. In particular, why do residents choose to spend their own money on expensive filters or commercial bottled water when the water coming out of their tap is, in almost all cases, perfectly potable? Are high-profile water disasters like Flint and Newark responsible? Are there other factors at play? With a viewpoint informed by social constructionism, environmental sociology, and social psychology, this study explores the social contours of the “tap gap.” It seeks to understand how different material and mental constructs –sociodemographic characteristics, trust, habits, salience, norms, and information – influence drinking water behavior. Using a mixed-methods approach, this study involved conducting and analyzing 130 face-to-face interviews conducted with residents of two apartment buildings (one mixed-income, one low-income) in Brooklyn, New York.The findings of this study reveal that social class and trust are both strongly correlated with the water people drink. These results have clear implications not only for researchers concerned with understanding the puzzling development of the “tap gap,” or environmental behavior more broadly, but also for government officials, water providers, building managers, anti-bottled water campaigners, and even individual water drinkers.