As you’re playing the Challenge, do you ever wonder why you see questions on words you have already mastered? It’s because the game is attempting to circumvent your brain’s natural process of forgetting.

In last week’s blog post, Can Forgetting Help You Learn? we introduced the idea that the brain is constantly discarding information. As if it’s following the housekeeper’s maxim to clean closets by giving away anything not worn in the last year, the brain appears to be fairly rigorous about getting rid of what it hasn’t recently used.

In the 1880s, a German scientist named Hermann Ebbinghaus began to study the way the forgetting process works. He memorized a bunch of nonsense vocabulary and then found, when he tested himself on it a few days later, he’d forgotten most of what he’d learned. (If you’ve ever crammed for a test, you’re probably familiar with this phenomenon.) Ebbinghaus then began tracking how long it took him to forget, and how his forgetting was impacted by reminders. Eventually, he created what he called a Forgetting Curve, which showed that the more often he was reminded of a fact he’d learned, the longer it would take to forget that fact. The brain, he determined, remembers only what it’s asked to remember.

And this is why you’ll see questions on words you already know when you play the Challenge. Asking you to remember while the knowledge is still relatively fresh assures that it will stay that way. Play often enough, and the words might remain lodged in your brain forever.