Misinformation is widely acknowledged as an important challenge for Western democracies. Yet, surprisingly little effort has been devoted to measuring the effects of various counterstrategies. We address this void by running a field experiment analyzing the causal effects of popular fact-checking videos on both believing and sharing misinformation among a diverse sample of American Twitter users (N = 1,600). We offer three empirical contributions: first, we demonstrate that fact-checking videos improve the ability to discern between mainstream and false news stories. Second, we show that they also improve the quality of news shared on Twitter. Third, we highlight the similarities and differences in the psychological antecedents of sharing and believing misinformation. Our paper aids the effort to design interventions for preventing or mitigating its negative effects.