The Day We Broke Milk

It started because Priya was bored of painting.

She'd done three pictures — a horse, something that was either a dog or a very confident cloud, and a sunset that was mostly brown — and pushed them aside with the air of someone who had exhausted an entire medium.

"What else can we make colours with?" she asked.

I looked around the kitchen. There was milk in the fridge, food colouring in the cupboard from a birthday cake we'd made three months ago, and washing-up liquid by the sink.

"What if," I said, "we made the colours move?"

She put down the paintbrush. Moving colours were better than regular colours.

The Setup

I poured a thin layer of full-fat milk onto a flat plate — just enough to cover the bottom — and let it settle for a moment. Then I handed Priya the food colouring.

"Drop one colour in. Just a small drop. Don't stir it."

She chose red, then added yellow, then blue, in three separate drops near the middle of the plate. They sat there, small and still, bleeding slightly at the edges into the white.

"It's just sitting there," she said. "Like the raisins did."

"Give it a second."

I dipped a cotton bud into the washing-up liquid and held it out. "Touch this to the middle of the milk. Gently. Just hold it there."

She did.

What happened next made her yank her hand back.

The colours exploded outward. Not slowly — immediately, violently, the reds and yellows and blues shooting toward the edges of the plate in long streaks, swirling around each other, the white milk churning underneath. It looked like a time-lapse of a weather system, or a galaxy forming, or a very small and beautiful disaster.

Priya made a noise that wasn't quite a word.

"Do it again," she said.

The Science Bit (Which Happened Naturally, I Promise)

We did it again. And again. Each time, the same explosion — the colours streaking outward the moment the soap touched the surface.

"Why does it do that?" she asked, which is the best question.

"What do you think soap does normally?"

She thought about it. "Gets things clean?"

"Right. How?"

"It... grabs the dirt?"

"Sort of. Soap is actually really good at grabbing onto fat. That's why it cleans greasy things — it latches onto the fat molecules and drags them away." I tapped the plate. "What's in full-fat milk?"

She looked at the milk. "Fat?"

"Fat. So when the soap touches the surface of the milk, it starts grabbing at the fat molecules and dragging them. And what do you think happens to everything else on the surface when the fat is being pulled in every direction at once?"

She watched the colours, still swirling slowly at the edges of the plate. "They get dragged too?"

"Exactly. The colours are just going along for the ride. They're not doing anything themselves — they're just showing you what the fat is doing."

She stared at it for a moment. "So the soap is fighting the milk."

"That," I said, "is a very good way to put it."

The Testing Phase

Once she understood the basic idea, she became immediately scientific about it, in the way children are when they've decided something matters.

She wanted to know: did it matter where you put the soap? (We tried touching the cotton bud to the edge instead of the middle. The colours moved differently — sideways rather than outward, which led to a long conversation about why.) Did it keep working if you added more soap? (We tried. After a while, the soap and milk reach a kind of truce and the movement stops. She described this as "the milk giving up.")

Then she asked the question I hadn't expected: "What about skimmed milk?"

We had some in the back of the fridge. We tried it.

The colours barely moved.

She looked at me. "Less fat."

"Less fat."

"So the soap has nothing to grab."

She grabbed the notepad she'd abandoned earlier — the one with the horse and the confident cloud — and started drawing what she called "the fat experiment," which was mostly a diagram of soap molecules attacking a milk molecule with small arrows showing the direction of battle.

Her mum came in, looked at the plate of swirling colour, looked at the diagram, and said, "Is that a scientific drawing?"

"It's a battle map," Priya said, without looking up.

The Next Day

She asked at breakfast whether butter would work, since butter is basically just fat and water. I told her that was an excellent question and we should find out, which we did, and the answer is: not really, because you can't pour butter onto a plate in a thin layer. But trying to figure out why it didn't work kept her occupied for twenty minutes.

She also told her dad that soap molecules are "basically tiny magnets for fat," which is not precisely accurate but is, honestly, a reasonable working model for a six-year-old.

The battle map made it onto the fridge. The confident cloud painting did not.

Your Turn

You need four things: a flat plate, full-fat milk, food colouring in a few colours, and washing-up liquid on a cotton bud.

Pour the milk, drop in the colours without stirring, touch the soap to the surface, and watch what happens. Then try skimmed milk and compare. Try touching the soap in different spots. Try adding a second drop of soap after the first one has stopped working.

The soap is doing everything. The colours are just showing you.

And if a six-year-old asks whether butter would work — say yes, let's find out. That question is worth twenty minutes of anyone's morning.