Fantastic that resources like this now exist. In some ways it seems to be reminding us about how we used to learn. Children spontaneously go back again and again to things that delight them (spaced repetition) and they switch activities when bored (Pomodoro). Unfortunately, perhaps as a result of schooling, or other hard knocks, the spontaneous impulse gets lost. Adults suffer from mixed motivations and seem to be fairly clueless about what they find genuinely interesting. It becomes difficult to approach topics playfully.