DuoLingo's Chess Icons Relate To Difficulty

DuoLingo launched chess lessons recently. When going through onboarding they use chess icons to indicate difficulty. 

It's a neat way to introduce fun into the app without being distracting. 

If you're a novice it subtly introduces the icons with some context. If you have some experience, you'll get the mapping and perhaps find it fun. 

And fun is part of the DuoLingo brand, this is just another excellent way they follow through on fun.