The immortal jellyfish, scientifically known as Turritopsis dohrnii, has a unique ability to revert to its juvenile form. 

This process, called transdifferentiation, allows the jellyfish to transform its mature cells into the cells of an earlier stage in its life cycle, effectively starting its life cycle anew. When faced with adverse conditions, injury, or old age, the immortal jellyfish can revert to its polyp stage, a younger form, through this process. 

During transdifferentiation, the jellyfish's cells can change type, allowing it to replace old or damaged cells with new ones. While this doesn't mean it replaces all of its cells simultaneously, it does mean that the jellyfish can effectively "reset" its biological clock, giving it the potential to live indefinitely under the right conditions. 

This remarkable capability has fascinated scientists and led to its nickname, the "immortal jellyfish."