Flashcards: read the question, try to remember, then reveal the answer and say how well you knew it. Easy cards come back after longer and longer gaps; missed cards come back soon. A few minutes a day is enough.
Check your understanding: every entry ends with open questions to answer in your own words.
Streak: a day counts when you review cards, finish a quiz, or spend some time reading an entry.
Every question and answer comes from the atlas entries and their sources.