Physicist Richard Feynman was a master explainer. His physics lectures are a model of clarity. His specialty is explaining one concept in the context of another concept the listener already understands.

In the video below, Feynman tackles the seemingly simple question: What is fire? This is a hard question for many of us to answer without referencing fire itself. Fire is burning. Fire is what happens when you light a match. Fire is a reaction. But aside from our existing knowledge of its effects (like flame), what is it and how does it happen? What else is fire like that we understand already?

In 1983, Feynman answered this question (among others) for the BBC on its Fun to Imagine series. He starts like so:

The atoms like each other to different degrees. Oxygen, for instance in the air, would like to be next to carbon, and if they're getting near each other, they snap together. If they're not too close though, they repel and they go apart, so they don't know that they could snap together.

It's just as if you have a ball that was [rolling and] trying to climb a hill and there was a hole it could go into. Like a volcano hole, a deep one. It's rolling along, and it doesn't go down in the deep [volcano] hole, because it starts to climb the hill and it rolls away again. But if you make it go fast enough, it'll fall into the hole.

So if you set something like wood in oxygen...there's carbon in the wood from a tree. And the oxygen comes and hits the carbon, but not hard enough. It just goes away again. The air is always [moving but] nothing's happening. If you can get it fast enough, by heating it up somehow...a few of [the atoms] come past, a few of them go over the top, so to speak.

So there you have it. If you understand what a volcano looks like and how a ball rolls under normal earth gravity, you have the start of this mental image for how fire works. Feynman's explanation manages to convey chemical behaviors in terms humans intuitively understand—the way a ball rolls—because that's the world we live in. Watch this for a delightful explanation:

The BBC has more clips (in better quality) at this slightly vintage website. You can also download the entire hour-long lecture from the Internet Archive.