Two years ago I was having breakfast with a man who was purportedly the most successful Jeopardy contestant ever — behind Ken Jennings (and the Watson supercomputer). As someone who is always interested in learning new things, I wanted to know how he was able to remember so much stuff.

“Have you ever heard of spaced repetition?” he asked me. “This is going to blow your mind.”

Since that moment, I’ve used spaced repetition nearly every day. I’ve used it to learn and memorize poems, speeches, people’s names, languages, and other random facts. I used to think I was just bad with names and birthdays, but I now realize all of that was just a story I told myself.

I learned that spaced repetition is a learning technique that relies on something called the spacing effect. The spacing effect says that it’s easier for you to learn or remember something if it’s repeated over a long period of time (a few months) rather than a short time (one night —i.e. “cramming”).

Think about how memory works for a second. We intuitively understand that there’s a difference between short-term and long-term memory. If I just try really hard for a few minutes, I might be able to remember my credit card number, a stranger’s name, or even a bunch of random facts for a quiz. The problem with this approach is that 24 hours later, I’ve already forgotten most of it. That’s called short-term memory.

Then you have long-term memory, which is where you store stuff like your name, your phone number, the address of the house you lived in when you were a kid, etc. Long-term memory is stuff that stays with you basically forever. Yeah, it’s possible to forget things in your long-term memory, but it takes a while (or something has to go really wrong with your brain).

But short-term memory and long-term memory are actually two ends of a spectrum. Everything that ends up in long-term memory has to start off in your short-term memory.

One of the most consistent ways that a new fact can move from short-term towards long-term memory is through repetition. Each time you recall something, it moves a little further into your long-term memory. In my limited understanding of how the mind works, I like to think it’s because the neural pathways to this new fact get stronger each time they’re used.

The interesting thing is this: the most effective amount of time to wait between repetitions in order to build long-term memory isn’t linear, but exponential.

When someone first tells me their name, I remember it for about five seconds. If I repeat it back to them or in my head a few times, I’ve bought myself probably another five minutes or so. At the end of the conversation, if I say their name as we say goodbye, I’ll remember it for another day. If I run into them the next day, then their name will stick with me for about a week, and so on.

In a paper on gradual-interval recall published by Paul Pimsleur in 1967, he hypothesizes the following intervals: 5 seconds, 25 seconds, 2 minutes, 10 minutes, 1 hour, 5 hours, 1 day, 5 days, 25 days, 4 months, and 2 years. Although the exact intervals varied by subject, Pimsleur’s intervals give you an idea about the exponential nature of memory retention.

One of the cool side-effects of this is that you can take a large set of facts (e.g. the 1000 most commonly used words in the French language) and introduce them in batches slowly (e.g. 10 per day). That way you only have to review each word every few months in order to remember them all indefinitely.

(The flip-side of this is: it doesn’t matter how awesome your teachers or your school was, if you don’t review each fact you learned at least every few years, you will forget them.)

There’s an app for that

The good news is that there are tools that will do the spaced repetition part for you. My Jeopardy friend told me about one called Anki that I still use to this day. (The desktop app is free.)

In Anki (and most other spaced repetition-based apps) you can either download or create your own sets of flashcards.

Every day Anki has you review only a handful of those cards and then you give it feedback about how easy it was for you to remember each card.

I’ll look at the card above and think “Oh that’s my friend Steve Klein.” Then I’ll click Show Answer.

In this case, I’ve seen this card a few times already, and it was pretty easy for me to remember. Not super easy, so I’ll click Good and I won’t see this card again for another 5.8 months. If I didn’t remember his name at all, then I’d click Again and this card would move back up to the beginning with 10 minute intervals.

I was able to use this system to learn the names and companies of the nearly 150 people in my Y Combinator batch within a few days.

My decks currently include: People, Quotes, and Speeches & Poems. I try to review each deck for about 10 minutes a day.

At first I got a little carried away and tried to memorize everything, including every language I had ever learned, information from books I was reading, and even the Major Mnemonic System for remembering long strings of numbers (don’t ask). The problem is that 10 minutes a day starts to add up as you add additional decks, so I’ve decided to only have three sets going at any given time.

In my people deck I manually create a new card for each new person that I meet. Because of my job, I meet a lot of new people in a given week. This used to lead to a lot of awkward second meetings where I couldn’t remember who someone was, where I had met them, or what they did.

Remembering names and things about people helps you build rapport. Once I memorized everyone’s name and company in my Y Combinator batch I was able to truly connect with people on a much more personal level.

The reason I’m memorizing quotes, speeches, and poems, is because it’s fun and useful to be able to pull out an appropriate and articulate quote or excerpt for any given topic. I think it will be helpful for doing impromptu speeches one day.

If you’re interested in learning more about spaced repetition, there are tons of great articles out there. One of my favorites is Derek Sivers’s post Memorizing a programming language using spaced repetition software. Another must-read is Piotr Wozniak’s 20 Rules of Formulating Knowledge.

In another post I’m going to write about the way I use Anki to memorize poems and other long bits of text. So stay tuned. And if you’re interested in other aspects of how I use spaced repetition, just ask below.

P.S. By popular demand, I’ve uploaded my Anki decks. Here’s the quote one and here’s the Speeches & Poems one.

Thanks to Chris Castiglione, Jake Heller, Nico Luchsinger, and Evan Walden for reading drafts of this.