There are immortal organisms!
Actually, biologically immortal. But it's still pretty exciting. Hydra are little water organisms. They're really weird in a number of ways, but one of the coolest things about them is that they do not age.
You know how kids heal really quickly from cuts, bruises, fractures, etc? And old people don't, of course. Most living things reach an age of maturity and from that point they experience a pattern of degenerating systems until a major system failure causes cessation of life.
However, the Hyda does not do this. It is eternally young in that the cells do not reach a "maturity" plateau and then degenerate.
I thought that was interesting
Of course, it is still possible to kill the hyda. It just doesn't age. Thus, biological immortality.
I figure this sort of ability wouldn't be good for humans to have