Ask HN: What metaphor did someone use that helped you “grok” something?

When I learned swimming as a kid, I struggled with coordinating how my legs were supposed to move. It seemed that I could either focus on my arms, or on one leg, but never on both legs, and much less on arms and legs together. Then my swimming instructor told me, "Move your legs just like a frog". That did it. That one metaphor took all the cognitive overhead away for me. I instantly "got" it.