How it works

Spaced repetition: the science behind every round.

Every player has a memory model for every question. The game knows what they have mastered, what they are forgetting and what they have never seen, and picks the right six questions for today.

First time
The question appears for the first time. The estimated memory is short: a few hours.
Correct
The interval before it comes back grows. The closer the player was to forgetting, the more it grows.
Wrong
The interval shrinks and the question returns soon, while the explanation is still fresh.
Mastered
Once the estimated memory passes seven days, the question counts as mastered and rarely appears.

The forgetting curve

In the nineteenth century, Hermann Ebbinghaus measured how much we forget as time goes by: memory of something new drops quickly in the first hours and more slowly after that. In 2015, Murre and Dros repeated the experiment, also with a single participant, and found similar results.

Our algorithm follows memory half-life models used in language learning apps: for each person and question, it estimates how long until the answer is forgotten. That is how knowledge retention holds up without long sessions.

Read the article on the forgetting curve

Schematic illustration. The gold dots are rounds; each one slows the next drop.

Levels and medals

Levelling up means mastering, not memorising.

Each level is a topic. To level up, players must answer most of the distinct questions in that level correctly.
Questions from earlier levels keep coming back, so they are not lost.
The six medals reward the total number of different questions each person has answered correctly.
Speed points, three helps per round (more time, 50/50 and swap the question) and practice streaks.