"Memory athletes" compete to see who can remember the most random numbers in an hour. Or else to memorise decks of playing cards. Memory training is big in China, where there are TV game shows for memory contests, and where parents pay good money to get their children trained in memorisation techniques. Everyone thought the Chinese were invincible, and that we were at the limits of what could be memorised - until 2019 when a group of North Korean teenagers arrived at the world championships, smashed a bunch of records, and returned to Pyonyang. Since then, nothing has been heard of them. What's their secret? And why does North Korea want to dominate the world in this obscure sport? Presenter/Producer: Jolyon Jenkins