I remember, for example, teaching a unit on “energy.” At some point, after I’d offered some unhelpful definition of energy—“the capacity for an object to do work”—and walked through its different forms, a student stopped me. “But what is energy?” It was clear she was asking me to name some thing, visible or not, that she could call “energy.” But the expectation that there’s something out there bearing the name energy is more confusing than helpful.

This recalls Feynman's amazing "energy makes it go" story from Surely You’re Joking:

For example, there was a book that started out with four pictures: first there was a windup toy; then there was an automobile; then there was a boy riding a bicycle; then there was something else. And underneath each picture it said, "What makes it go?"

I thought, "I know what it is: They're going to talk about mechanics, how the springs work inside the toy; about chemistry, how the engine of the automobile works; and biology, about how the muscles work."

It was the kind of thing my father would have talked about: "What makes it go? Everything goes because the sun is shining." And then we would have fun discussing it....

I turned the page. The answer was, for the wind-up toy, "Energy makes it go." And for the boy on the bicycle, "Energy makes it go." For everything, "Energy makes it go."

Now that doesn't mean anything. Suppose it's "Wakalixes." That's the general principle: "Wakalixes makes it go." There's no knowledge coming in. The child doesn't learn anything; it's just a word!