Nothing special about duolingo, just that it gamifies something very dry and makes it fun, and won't let you progress until you're actually good enough (e.g. I

Nothing special about duolingo, just that it gamifies something very dry and makes it fun, and won't let you progress until you're actually good enough (e.g. I would pronounce 'robes' [french for 'dress'] and it wouldn't let me pass until I pronounced it 'hhrobes'). 'Duolingo' is just a good (but as you point out, not the only) example of an app that gamifies, in the right ways, an otherwise dry topic.