What it teaches
The planets in order from the Sun, and the facts that make each one memorable — the biggest, the ringed one, the red one. Also what the Sun actually is, which surprises most children: a star, and not a special one.
How it works
A question appears with answers underneath. Three levels change the length of the round and the number of answers offered.
Wrong answers come back through the daily review, which suits this subject: space facts are exactly the kind a child half-remembers.
What to look for
This is the subject children most often already know something about, and the one where a wrong answer is most likely to be a half-remembered fact rather than a guess. Worth asking where they heard it.