This post was written by Connor Balough

If you’ve logged on Facebook in the past month, aside from liberals crying, you’d have seen the question “Which Tank fills first?”

A plumber spoke out, and scientists agree.


But first, why is it so popular, and why does nobody agree.


Psychologists have the answer:


So, did you get 1 or 9? We’ll get to the “correct” answer in a moment.

But first, why do we get so riled up about these problems? People don’t usually get into fistfights at the bar over arithmetic, but these math threads are spectacularly vitriolic. A couple of factors are at work in these math debates, according to Robert Glenn Howard, a social psychologist at the University of Wisconsin–Madison who specializes in Internet communication and folklore.

For one thing, the whole point of Facebook and other forums is to provide a place for discourse and debate. Yes, there are your cousin’s new-baby pictures, and the opportunity to stalk a crush, but really, people go to social sites to say stuff. And argue about it. “People are already primed to engage in pretty intense deliberations, and that can bleed over into the way they play games,” Howard says.

And that’s exactly what these problems are: games. “Humans have used riddles as a form of play since ancient times,” Howard says. “And sometimes people can get competitive and wrapped up in it.” People use puzzles to show off their smarts, make others feel subordinate, and enjoy telling the story of the game later (as I’m doing right now).

Of course, the fervor with which some people debate basic arithmetic may be a proxy: There’s less at stake in a math debate than a potentially friendship-ending political debate. Arguing over multiplication may even be a way to make a subtle political point, using others’ “wrong” answers to reinforce a broader worldview, such as that the United States has poor math education.



Tank 3 will be the the first to mostly fill. Tank 4 won’t start to fill until tank 3 is full to the level of pipe that enters 4. Nonetheless, tanks 3 and 4 will overflow simultaneously because their top edges are aligned, and are lower than either of the other two tanks. So the answer is 3 and 4 together.

It would be a fun experiment to recreate using e.g. paper cups, straws, and tape.

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