This paper presents a video-based field study of the Reactable, a tabletop tangible user interface for music performance, in a hands-on science centre. The goal was to investigate visitors' social interactions in a public setting. We describe liminality and cross-group interaction, both synchronous with fluid transitions and overlaps in use between groups, and asynchronous. Our findings indicate the importance of: 1) facilitating smooth transitions and overlaps between groups, and 2) supporting not only synchronous but also asynchronous group interaction. We discuss the lessons learned on how best to enable liminal situations in the design of interactive tabletops and tangible user interfaces for social interaction and particularly collaborative tangible music in public museum settings.