Refresh is important because at some point you’re going to forget what you learned. In fact, scientists have built models called forgetting curves, which predict how and when you’ll forget learned information. However, forgetting curves differ from person to person due to a variety of inputs, such as strength of prior knowledge and time spent learning. Our system adapts to you and your unique knowledge levels. Based on your interactions in the program, the system can form an idea of when you are going to forget, and encourages you to refresh at the best time—just as your memory is starting to fade.