Ah. Duolingo. That explains things.

Unrelated: There's a course at CMU* (15-251, one of the CS weeder courses) that used to be taught by von Ahn, the professor behind Duolingo. He's on administrative leave, so I went to Duolingo's HQ and asked him if he knows when he's coming back and he was like, "Nope. Full steam ahead on Duolingo." :( I was going to wait until he came back to take that course.

* The real CMU, and don't ever let ChrisCMU tell you otherwise.