Explain It to a Ten-Year-Old: An Old Habit for Finding the Gaps in What You Know

A while ago, at a family lunch, my niece asked how the fridge keeps the milk cold. I had an answer ready. "It moves the heat out," I said, rather pleased with myself.

"Out where?" she said. "And how?" And then, after my second answer, the one that really did me in: "But why?"

I opened my mouth and found I had a sentence but not an explanation. I had learnt the phrase "it moves the heat out" somewhere along the way and had been carrying it around like a coin I'd never spent. I'm grateful to her. She found the gap in about ninety seconds, and she was kind enough not to look triumphant.

That is the whole technique, and it answers the question in the title. To check whether you understand something hard, explain it in plain words to someone who has no jargon to lean on, such as a curious ten-year-old. Wherever you stall, hand-wave or say "somehow", you have found a gap. Fixing the gaps is the learning.

Where does this habit come from?

It goes by several modern names, but the habit is much older than any of them. The earliest careful description I know of is in Plato's dialogue Meno, written in the fourth century BCE.

In it, Socrates draws a square in the dirt, two feet on each side, and asks a young servant of Meno's household how to make a square with double the area. The boy answers confidently: double the side. Socrates lets him try it. A four-foot side gives a square of sixteen square feet, not eight. The boy tries three feet, which gives nine. Then he admits he doesn't know.

Socrates points out, gently, that this is progress. The boy now knows he doesn't know, which is a better place to start than false confidence. With a few more questions the boy sees that the right square is built on the diagonal of the original one.

Two cautions are fair here. Historians are unsure how far Plato's Socrates matches the real man, because Plato wrote the dialogues and we have no recording. Readers have also argued ever since about whether Socrates leads the boy too much. But the shape of the scene is clear. A confident answer meets a plain question, the answer wobbles, and the person sees where their understanding actually ends.

What did Seneca and Franklin add?

Socrates was mostly the questioner. The later step was to turn the questioning on yourself, or to see teaching as a way of learning.

In one of his letters to Lucilius (Letter 7), the Roman writer Seneca notes that people learn while they teach. The thought is simple, and it matches my experience at that lunch. Having to put an idea into words for someone else makes you test it in a way that quietly nodding along never does.

Benjamin Franklin gives a gentle, practical version in his Autobiography. As a young man he read an account of Socrates' conversations and liked the method of asking modest questions. He also adopted a habit of avoiding blunt, certain wording, and of saying things like "I imagine" rather than "certainly". What I take from this is that asking plain questions of an idea, and of yourself, works best in a gentle, curious spirit. It isn't about catching anyone out.

Over the centuries the habit has turned up in many places. Teachers use it. Programmers have a well-known version in which you explain your code, line by line, to a rubber duck on your desk, and the bug often shows itself halfway through. The duck says nothing. The explaining does the work.

Why do gaps show up when you skip the jargon?

A technical word is a label for a chain of reasoning. You can use the label correctly, in the right sentence at the right moment, without owning the chain behind it. Other people nod, you feel fluent, and nobody finds out, including you.

Plain words don't allow that. If you may not say "refrigerant cycle" or "thermodynamics", you have to say what actually happens first, then what happens next, and why. Each "because" is a link in the chain, and a missing link has nowhere to hide.

A ten-year-old is good for this because they are honest about being lost. They don't say "ah, right" to save your feelings. And they keep asking "but why?", which is exactly the question the chain needs to survive.

A worked example: how does a fridge work?

Here is the method, using my niece's question. The steps are simple: say it plainly, mark where you stall, go and fill only those gaps, then say it again.

My first attempt, in plain words: "The fridge takes heat from the inside and puts it into the kitchen."

That sentence is true. The trouble began with the next questions, and I had three gaps:

1. Why would heat go the "wrong" way? Heat flows by itself from warmer things to cooler ones, never the other way round. The inside of a fridge is cooler than the kitchen, so moving heat out of it is like pushing water uphill. I had no idea what did the pushing.
2. What carries the heat? I knew there were pipes. I couldn't say what was in them.
3. How can a pipe inside the fridge be colder than the food, while a pipe at the back is hotter than the kitchen? One loop of pipe doing both jobs made no sense to me.

I went and read, just about those three things. Here is what I put together. Details differ between models, so treat this as the usual household type.

The loop, in four stops. A sealed loop of pipe holds a substance called a refrigerant, which is liquid in some parts of the loop and gas in others. It travels round and round.

• Stop one, inside the fridge. Cold liquid refrigerant flows through pipes and boils into gas. Boiling takes in heat, so it draws heat out of the air and food around it. Your wet skin feels cool in the wind for the same reason: evaporating water carries energy away with it.
• Stop two, the compressor. This pump squeezes the gas. Squeezing a gas warms it, as you can feel in a bicycle pump after some hard pumping. The gas is now hotter than the kitchen. This is the "push uphill", and it is why the fridge needs electricity.
• Stop three, the coils at the back or underneath. The gas is hotter than the room, so heat flows from it into the room, which is the natural direction. As it loses heat, it turns back into liquid.
• Stop four, the narrow tube. The liquid is forced through a very narrow passage, and its pressure drops. It arrives cold and ready to go round again.

Why does the pressure drop make it cold? This was where I'd have been most tempted to hand-wave, so it is worth doing slowly.

A liquid's boiling temperature depends on the pressure around it. Lower the pressure and it boils at a lower temperature. When the warm, high-pressure liquid suddenly meets low pressure, it is far above its new boiling temperature, so some of it flashes into vapour at once.

Turning liquid into vapour takes energy. The vapour carries that energy off, and the energy comes from the liquid that stays behind. That remaining liquid therefore cools, until it settles at the new, lower boiling temperature. The result is a cold mixture of mostly liquid and a little vapour. Then it flows through the pipes inside the fridge and takes in heat there, which brings us back to stop one.

So my three gaps had answers. The compressor does the pushing. The refrigerant carries the heat. One loop can be cold in one place and hot in another because the pressure differs from place to place.

The ten-year-old version, which I tried on my niece afterwards: "A fridge is a heat-mover. Inside its pipes is a liquid that boils very easily. When it boils inside, it soaks up heat from your milk. Then a pump squeezes it so it gets hot enough to pour its heat out into the kitchen, and it turns back into a liquid. A tiny tube lets it expand and go cold again, and round it goes."

She said, "So it's like a sponge for warmth." I think that is a good picture, and she came up with it, not me. I'm still learning how far the sponge idea stretches, but it carries the main point.

What's the common misconception here?

There are two, and the first is gentler than it sounds. We say a fridge "makes cold air", and in everyday talk that is fine. But it is more accurate to say it moves heat. Coldness isn't something the fridge manufactures. Heat is energy, and the fridge shifts it from one place to another.

That also gives a nice thought experiment. Suppose you leave the fridge door open in a closed, sealed kitchen. Will the room cool down? Going by the loop above, no. The fridge moves heat from one place in the room to another place in the same room. And the electricity driving the compressor ends up as extra heat. So the room warms slightly. Our everyday intuition says the opposite, and the plain-words explanation puts it right.

The second misconception is more general: the feeling of understanding is not understanding. Familiar words feel like knowledge. I had said "it moves the heat out" for years with total comfort. The comfort was real, but it didn't tell me anything about whether I understood.

How can you try this yourself?

Here is the version I use now. It's gentle, it costs nothing, and it needs only a notebook or an obliging relative.

1. Pick one small question. "How does a fridge work?" is better than "How does physics work?"
2. Explain it in plain words. Write it or say it aloud. If you use a technical term, explain it with simpler words straight away.
3. Mark every stall. Note each place where you said "somehow", "basically", "it just does", or went quiet. Don't be hard on yourself. Every one of those marks is a gap you found, and that is the point of the exercise.
4. Look up only the gaps. You don't need to re-read the whole subject, only the three or four places that wobbled.
5. Explain it again, shorter. If you can add a comparison, like the sponge, check that it doesn't mislead anyone.

Expect your first attempt to be muddled. Mine were. My early drafts of this very explanation had the compressor and the condenser in the wrong order, and I only noticed when I tried to say the loop aloud. Results vary too. Some topics fall into place after one round, while others need several, and a few I've had to put down and come back to.

I should also say where my own explanation stops. Some fridges, such as certain caravan and gas-powered models, use a different method, called absorption. They don't use a compressor at all. I haven't made a proper study of them, so I'll leave them for another day.

What is still worth wondering about?

Socrates' young servant lost his confidence and gained something better. My niece gave me a similar gift, with a smaller square and a much colder glass of milk. One thing keeps nagging at me. If one honest "but why?" can open a gap in an idea I've repeated for years, how many of my other well-worn phrases are coins I've never spent? I don't know. But I've started asking, kindly, and I'd like to hear what you find when you try it on something you think you know.