You’ve heard of the Peter principle? It’s the idea that in every organization workers are promoted until they achieve a job they can’t do. Then they stay there.

My job involves a lot of meetings. I’m not very good at meetings. My mind wanders to topics not remotely related to the meeting.

But I think I am pretty good at teaching. Today I got a rare chance to teach. I was stationed at a cart with a cloud chamber. A group of fifth graders came over, three girls, one boy, and their adult chaperone. We talked about atoms, how amazingly small they are, how separating water into hydrogen and oxygen (always exactly 2 hydrogens for 1 oxygen) gives a clue about the reality of atoms. Then we looked at the cloud chamber.

Inside the cloud chamber is a lantern mantle coated in thorium. When the thorium decays, it releases an alpha particle. Several more alpha and beta decays transmute the thorium finally into lead. The ethanol in the atmosphere forms a cloud behind the decay particles, showing their paths with white, wispy lines.

Here’s a great movie of a cloud chamber in action.

As my learners watched the paths of these bits of exploded atoms, we talked about what was happening and why. Why did one atom decay today, in this moment, after waiting silently for billions of years? Why did the ethanol form those wispy clouds? What happened to the particles afterwards?

I talked to my learners about helium, about how virtually all the helium on Earth comes from radioactive decay, and how every time they hold a helium balloon in their hands what they’re really holding are the leftover bits of exploded radioactive atoms.

Then we talked about where thorium comes from, how, billions of years ago, a giant star exploded, and in the explosion energy was stored in a new kind of atom, an atom called thorium. We talked about how that thorium atom drifted in space for maybe billions of years, until it finally found itself swept up in the formation of a young planet we would one day call Earth. We talked about how when that atom exploded it released, finally, that stored-up star energy, captured there so many years ago.

The boy asked, “Don’t exploding stars form nebulae?”

“That’s right,” I said, “Those clouds around ancient stars contain all the elements the star forms in its lifetime, including the elements that make up you.”

And now I looked one of the girls in the group, the quietest one, directly in the eyes and said, “The very atoms that make up you were born billions of years ago in a giant star. You are one of the things stars can make.” And the girl smiled.

And that’s why I do what I do.