Scientists decapitated some trained flatworms. They remembered their training after their heads grew back!

Flatworms are tiny creatures that can usually only be seen under a microscope. Scientists spent two weeks training them to get food by moving across a rough floor of a petri dish. Other worms were introduced, and the ones that had been trained on the rough floor were more comfortable with their environment and got to their food faster than the newly introduced worms. The trained worms then got their heads cut off!

Amazingly, when their heads grew back, the worms retained their training! After just one 'reminder' session, they reached their food much more quickly than worms who had never been trained at all. Despite all of this research, scientists actually have no idea how this is possible!

(Source)