The Rainbow in a Jar

It started because Lily wanted to know why oil floats.

She'd watched me pour salad dressing too fast and seen the oil pool on the surface, and she'd asked about it at dinner, and I'd said "density" without really explaining it, and she'd filed that away in the way she files things away - quietly, for later.

Later turned out to be a Saturday morning.

"You said density," she announced, appearing in the kitchen. "Can we see it?"

I looked at the time. I looked at the jar of honey on the counter.

"Yeah," I said. "Go get the food colouring."

The Setup

The trick with density is that you can see it but you can't touch it. The molecules in honey are packed tightly together - dense, heavy, slow. The molecules in rubbing alcohol are loosely packed - light, quick, barely there. Put them in the same glass and they'll arrange themselves in order of weight, heaviest at the bottom, lightest at the top, without you having to do a thing.

We lined up everything we had: honey, dish soap, water with blue food colouring, vegetable oil, and rubbing alcohol with a drop of red in it.

"Which one do you think is heaviest?" I asked.

Lily picked up the honey jar and hefted it. "This one."

"So that goes in first."

She poured it slowly into a tall glass. It moved like honey moves - thick, amber, taking its time.

"Now what?"

"Now we pour the next one very slowly, over the back of a spoon. If we rush it, they'll mix and we'll lose the layers."

She looked at the spoon. She looked at the dish soap. She is six and patience is not always her strong suit, but something about the gravity of the spoon technique made her careful.

She was very, very careful.

The Watching Part

Layer by layer, the glass filled up. Honey at the bottom, golden and still. Dish soap above it, faintly blue. Then the coloured water, settling into its own band. Then the oil - which refused to do anything dramatic, just floated there, clear and self-contained. Then the red rubbing alcohol on top, the lightest of all, sitting on the surface like it owned the place.

Lily held the glass up to the window.

Five layers. Each one separate. The colours sitting in their own bands like a flag, or a sunset, or something made by a person with very steady hands and a lot of spare time.

"They're not mixing," she said. She sounded like she didn't quite believe it.

"They won't. Each one is staying where the weight says it should be."

"What if I shook it?"

"Then it would mix and you'd have a brownish mess."

She considered this. Then she set the glass back down on the windowsill, carefully, and took a step back to look at it.

"I'm not going to shake it," she said.

The Questions

"Is everything like this?" she asked. "Everything has different density?"

"Everything has some density. Whether the differences matter depends on what you put together."

"What about me and water? Am I more dense than water?"

"Roughly the same. That's why you float but only just."

She thought about this for a long time. Then: "What about rocks?"

"Denser than water. That's why they sink."

"What about boats? Boats are heavy and they float."

I paused. "That's - actually a really good question. Do you want to work that one out?"

She did. It took most of the morning. We looked up displacement, drew diagrams, argued about whether a hollow rock would float (she said yes; she was right). The rainbow jar sat on the windowsill the whole time, holding its layers, patient as anything.

Your Turn

You need a tall clear glass and five liquids: honey, dish soap, water (add food colouring to make it visible), vegetable oil, and rubbing alcohol (add a different colour).

Pour them in that order, each one slowly over the back of a spoon. Don't rush. Watch each layer settle before you add the next.

Once you've got your rainbow, try dropping small objects in - a grape, a coin, a piece of cork - and see where each one stops. What does that tell you about its density?

And if a six-year-old asks why boats float, let them work it out. That question is worth the whole morning.