Watching the smooth movement generated by hundreds of fish as they swim in unison is truly mesmerising. But it’s not only its sheer beauty that makes it so hard to look away, for scientists, it’s also the fact that its emergence is so difficult to explain. In an article published today (February 13, 2017) in the scientific journal PNAS, researchers from Champalimaud Centre for the Unknown (CCU), in Lisbon, present a surprisingly simple rule that explains how complex patterns of collective movement emerge in zebrafish as they develop from recently hatched larvae into adults.