People are talking about hippos this week, at least in part because the Cincinnati Zoo’s beloved baby hippopotamus, Fiona, is now three months old—a milestone that seemed uncertain when she was born prematurely in January. Fiona’s doing great—so great that she’s “a little bit dangerous to actually cuddle and snuggle” anymore, the zookeeper Jenna Wingate told local reporters.

Which reminds me that wee Fiona will eventually, if she continues to thrive, turn into a grown-up hippo. And grown-up hippos are not—I repeat, not—to be trifled with. Consider, for example, this video, which my colleague Ed Yong shared with me yesterday:

Look at that hippopotamus go! After sharing this video on Twitter, I got several perplexing responses. Comments like this: “Not bad for an animal that doesn’t swim,” and “And … they can’t even swim!” There is even, someone told me, a children’s book about this: Hippos Can't Swim: And Other Fun Facts. As a long-time skeptic of “fun facts,” I obviously had to know more.

The San Diego Zoo’s fact sheet about hippos wasn’t exactly clarifying: “hippo limb muscles are for powerful propulsion through water, but not swimming.”

What is swimming, if not using one’s limb muscles for powerful propulsion through the water? “Well, a boat doesn’t swim,” my colleague Molly Ball, pointed out. But hippos don’t have engines, or propellers, or sails. So how, exactly, do they charge through the water so impressively? After contacting half a dozen zoologists and wildlife parks, I finally had my answer.