Teach it back: the best test of understanding
In Teach it back the roles flip: you explain the idea, and Why Tho? plays someone hearing it for the first time. Its curious-beginner questions show the gaps a quiz would miss.
Why explaining works
If you can explain something simply, you understand it. If you get stuck, you've just found exactly what to learn next. Teachers call this the protégé effect.
How a round goes
Tap Explain it back after a lesson. Explain in your own words; the beginner asks what a real beginner would ("but what happens to the salt after that?"). A small tracker shows how many of the key ideas you've covered, like 3 of 4 ideas. Tap Done whenever you like.
Like I'm 5
Like I'm 5 starts the same round with a five-year-old instead: very short sentences, lots of "but why?", and lost at the first big word ("what's a chlorophyll?"). If you can explain it to them, you really get it.
No character, no grades
The beginner is just a way of answering, with no name or face, and there's no score: the point is to find the gaps, not to pass.