Until Leonard Hayflick came along, everyone thought cells were immortal, that they’d divide over and over again, forever. Hayflick torpedoed that theory and proved that there is limit. A very predictable limit: a magic number. To thank him, science textbooks everywhere now refer to that as ‘the Hayflick limit.’

While Hayflick’s discovery may seem like a buzzkill for those in search of immortality, fear not! Rules were meant to be broken, and Hayflick, now a Professor of Anatomy at the University of California, points out how one might tinker with this limit in the lab, and the potential dangers of doing so. The limit is different, it turns out, for different animals. There are tortoises that live for centuries, and lobsters that seem to be ageless.