The Immortal Jellyfish

Here is how Turritopsis pulls it off. When the adult is injured, starving, or just old, it sinks to the seafloor and melts back into a juvenile polyp — as if a butterfly turned back into a caterpillar.

The process is called transdifferentiation: finished adult cells switch jobs, with muscle cells becoming nerve cells and so on. The animal rebuilds itself from the inside out and can repeat the loop again and again. Predators and disease still kill most of them — but aging, the clock that limits every other animal, is not what stops this one.