Incivility in online discussions is an ongoing concern in academic and popular circles alike. Although social media offers the possibility for meaningful discussion, research has identified many barriers to this potential including disrespectful interactions, echo chambers, misinformation, and participation gaps. Most such scholarship focuses on just one topic of discussion, however. By comparing two domains of exchange, the authors are able to examine whether and how the subject of conversation may influence online experiences. The authors analyze interviews with 45 adults from across the United States about their experiences discussing science and religion on social media. People approached the two topics differently, which influenced whether they contributed to related conversations. The intrusion of politics into conversations across both topics broadly limited participation. Curiosity, knowledge, and interacting in private groups or with strangers encouraged joining discussions. Understanding participation dynamics across topic domains is fruitful for future research on the social media public sphere.