Some cells do manage to maintain youthful flexibility. Stem cells are the focus of intense scrutiny by researchers hoping to exploit this quality to repair damaged tissues and organs. Adult stem cells could perhaps be coaxed to form a larger range of tissues than they already do, more like a doctor retraining to be a lawyer than a sumo-wrestler retraining to be a ballerina. Or scientists, like domineering parents, could push embryonic stem cells, which are more flexible still, to become the cell type of their choosing.

As well as helping us to develop stem cell therapies, understanding how these fickle cells keep their options open yields new insights into embryonic development and how tissues like skin regenerate. It also shows us what happens when these things go wrong. There is growing evidence, for example, that faulty stem cells lie at the heart of many cancers. Learning more about how the process of specialisation goes awry in such cells could lead to new targets for treatment.

Epigenetics is a key controller of stem cell flexibility. New research suggests many such cells are primed to embark on a particular career in life, but are held back from specialising until the time is right. This feature, called bivalency, could help explain a long-standing mystery of developmental biology: how do cells stay “stemmy”?