Spaced repetition is an observed phenomena in which people are more likely to remember information if they study it in dynamically spaced intervals, as opposed to regularly spaced intervals. In practice, we can take advantage of this with a spaced repetition system (or SRS). Say you have a deck of digital flash cards. For each card, you are shown one side and prompted to type the corresponding answer that would be on the back side. The SRS keeps track of how well you know a card based on how often you answer it correctly. If you answer a card correctly, the SRS will show you that card less often until it stops showing up. If you answer incorrectly, the card will be shown more often.