Most of our cells contain an immobile primary cilium. The 'skeleton' of the cilium consists of microtubule doublets, which are 'pairs' of proteins essential for their formation and function. Scientists have developed an in vitro system capable of forming microtubule doublets, and have uncovered the mechanism and dynamics of their assembly. Their study reveals the crucial role of tubulin, a real building block, in preventing the uncontrolled formation of ciliary structures.