Hi!

This week, I learned something interesting that didn’t fit into the normal Monday through Thursday format but still wanted to share it. I guess that’s what Fridays are for! Let’s jump right in. I’ll bullet this out to make it easy to read, step by step.

1. Pick any four digit number with at least two unique digits — so, you can’t have 1111 or 2222 or 3333 etc. as your selection.
2. Take that number and rearrange the digits from highest to lowest — we’ll call that the High value.
3. Then take the number and rearrange the digits from lowest to highest, with any zeroes coming at the front. We’ll call that the Low value.
4. Subtract the Low value from the High Value. That’ll give you a new number, which we’ll call the New Number. (Creative, huh?)
5. Repeat steps two through four with the New Number until the result you get is the same as the New Number.

If you follow that process, you’ll always — always — send up with 6,174.

Let’s try it with my birthday, 12/13. (Yes, it’s a month away, and I collect Mets caps if you want to get me something, size 7 7/8. Although I don’t know how you’d send me one.)

1. I chose 1213.
2. My High value is 3211.
3. My Low value is 1123.
4. 3211-1123 equals 2088.
5. My new High and Low values are 8820 and 0288, respectively. Subtracting High from Low gives me 8532.
6. My new High and Low values are 8532 and 2358, respectively. Subtracting High from Low gives me 6174. (We got there quickly with that one!)
7. Just to check: The High and Low values for 6174 are 7641 and 1467, respectively. Subtract High from Low and the result? 6174.

Pretty neat, right? But also, mathematically just kind of random. The number 6,147 is called Kaprekar’s constant, named after Indian mathematician D.R. Kaprekar, who discovered the idiosyncratic property in 1955. There’s no cool math proof to show why this works, unfortunately (see this Stack Exchange thread for more).

So again, there’s no story behind this or greater learning — so I don’t have a showstopping ta-da moment to end with. I hope you still find this fun and interesting! Let me know if you end up sharing it with anyone in your lives!

The Now I Know Week In Review

Monday: Does This Expensive Coffee Taste Like Poop?: I don’t drink coffee (or eat poop) so I don’t know, personally.

Tuesday: How the Civil War Decided What Pie You Eat on Thanksgiving: In case you were wondering, here are my pie rankings, as of right now and very subject to change: Pecan, Apple, Cherry, everything else. (This has nothing to do with Tuesday’s story, though.)

Wednesday: The Tool That’s Made to Be Broken: The metal thing at the end of your tape measure isn’t loose. It’s supposed to be that way.

Thursday: The Baby Deterrent That Backfired: Boredom is a worse curse than teenage parenthood, apparently.

Long Reads and Other Things

Here are a few things you may want to check out over the weekend:

1) “The Runaway Monkeys Upending the Animal-Rights Movement” (New Yorker, 45 minutes, November 2025). This one is kind of political, and I went back and forth about sharing it as a result, but it’s weird politics.

2) “How a humble weed became a superstar of biology” (Knowable Magazine, 17 minutes, November 2025). A scrappy little weed once ignored by botanists became the unlikely superstar that transformed modern plant science. Here’s how Arabidopsis thaliana went from roadside nuisance to the organism that unlocked the secrets of how plants grow, adapt, and survive.

3) “Back to BASIC—the Most Consequential Programming Language in the History of Computing” (Wired, 8 minutes, July 2024). BASIC was my first (and really only) programming language, so I had little hesitation to click on this one.

Have a great weekend!

Dan