This weekend, 50,000 runners will toe the line at the New York City Marathon (the world's largest 26.2!)—and we’re guessing that, throughout their training, many of them could relate to the narrator in the quick flick below: “I’m 15 miles into this run,” he says, groaning. “I have another seven to go. I’m losing steam, my legs are cramping, I can barely breathe. I don’t know if I can make it another mile!”

While we hope the marathoners feel better than that on Sunday morning, we’ll admit it: We’ve been there.

The video, from the American Chemical Society, goes on to explain the science behind why we “bonk,” “hit the wall,” or, if we’re lucky, catch the drug-like runner’s high. (Hint: It’s got a lot to do with proper training.)

If you’ve ever wondered why it’s important to carbo-load, what that burning feeling in your legs really means, why you sweat so much, and whether you can drink too much water, the answers are here.

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Whether you’re racing this weekend or not, give it a watch. Then get out there and go chase that dopamine rush—through NYC’s five boroughs or wherever you love to pound the pavement. Good luck, marathoners!