The jellyfish that grows young to cheat death

Turritopsis dohrnii is a tiny jellyfish, about the size of a little-finger nail, that lives in temperate seas around much of the world. Its trick is one of a kind: when it grows old, falls ill or goes hungry, it can revert to its juvenile stage.

Its adult cells reprogram themselves and the jellyfish turns back into a polyp, the bottom-anchored stage that begins the life of almost every jellyfish. From there it grows and reproduces all over again. In theory it can repeat the cycle indefinitely, which is why it's known as the immortal jellyfish.

Mind you, it isn't invincible: a fish can eat it or a disease can kill it before it reacts. But left in peace, it has found a way to dodge old age by rewinding its own life.