Why Is 9 + 10 = 21? The Math-Bending Vine Explained

What's 9 + 10? If the mathematically correct answer "19" is the first thing to pop into your mind, congratulations, you probably spend a perfectly healthy amount of time online.

Still, if you find yourself confused about why way too many people answer "21" to this question, you're in the right place. Find out all about the famous Vine and meme that changed the rules of math forever in our explainer below.

Where Does "9 + 10 = 21" Come From?

In June 2013, Vine user @DREHUPEMSU @WESTROSECRAN posted a clip in which he calls a boy, likely his brother, stupid. As the boy protests, the Viner asks him what is a sum of nine and ten, with the boy failing to do a correct calculation and answering "21," after which he immediately gets called stupid again. The Vine went viral on the app, gaining over 700,000 likes and 30 million loops.

- You stupid.
- No, I'm not.
- What's nine plus ten?
- Twenty one.
- You stupid.

The meme lived on in the Internet's memory for six years before in 2019, thanks to the combined efforts of iFunny, Twitter and Instagram shitposters, the meme came back in force. This time, users edited the Vine in every imaginable way, starting from combining it with other popular formats, to altering voice lines for hilarious results, to using the soundbites in the 21st Century Humor montages.

Why Does "Nine Plus Ten Equal Twenty One?"

Already in 2016, people were confused by the joke as questions about the meme appeared on Quora. As the meme came back in 2019, the joke confused increasingly more people who were not in the know.

While 9 plus 10 does not actually equal 21, it's simply fun to claim so as a reference to the meme. It's also an inventive way to distinguish a real human from a bot in online chats — try it out!

9 + 10 = 21 Usage In Memes

In memes, the Vine has been used in a variety of formats from caption edits to appearing in rapid-fire 21st-century humor montages. At this point, the meme has long since reached a point of overuse, but it doesn't mean that if you have an original idea for it you shouldn't go for it!

For Instance, in 2021, a Twitter gimmick account "Days until 9/10/21" was created, counting down days until September 10th, 2021, or the 9/10/21 day. The account was a massive success, reaching over 118,000 followers and some of its tweets getting past 150,000 likes.

TODAY IS 9/10/21 pic.twitter.com/ocvFWunp3b

— Days until 9/10/21 (@TwentyOneCount) September 10, 2021

For the full history of '9 + 10 = 21' be sure to check out our entry for more information.