Density (as told to a five year old)

In response to Mommazen's discussion of "teachable moments" in parenting, I think of my trip this morning to drop off the Painter at kindergarten. A delightful morning greeted us, with grey skies and heavy drops of moisture on the grass, the ground, and the car. (Recall the recent awful wildfires here. Grey skies and rain are heaven sent!)

Me: They call this dew. When the moisture in the air condenses on cold mornings, we call it dew.

Painter: [Looking at me intently, as he gets in his car seat].

Me: See... um... on cold mornings... let's see. When it's cold outside, the moisture in the air condenses, the moisture in the air, that's in the form of a gas, becomes a liquid, water... and since water is denser than air it forms on the ground and surfaces like the top of the car. See, everything in the world is made up of atoms.

Painter: [Amazingly, still intently listening at this point, as I fumble to clarify my own somewhat vague understanding, trying to remember my undergraduate paper on the topic of "caloric".]

Me: Atoms are these little tiny particles, they're so small you can't see them, except with a microscope. We call them microscopic. So everything is made up of these tiny atoms, and well, when the temperature is warmer, they're further apart and moving faster. But when the temperature is lower, these atoms move closer together, and become denser. [Desperately trying to figure out whether temperature is the cause or the result of this state of affairs. Remembering that the 18th century theory of caloric as a substance certainly served to simplify this question.]

Painter: What's that mean?

Me: Density?

Painter: Yeah.

Me: Um... okay, some things are heavier than other things, right?

Painter: Yeah.

Me: Well, some things that are even the same size, can have different weights.

Painter: No... I think everything that's the same size weighs the same thing.

Me: Okay, well, have you ever picked up a crumpled ball of paper? [Knowing of course that he has.]

Painter: No.

Me: Have you ever picked up a rock?

Painter: Yeah.

Me: Have you ever picked up a leaf that was the same size as a rock?

Painter: No.

Me: [So he's going to be difficult.] Alright, have you ever picked up a ball? Like a tennis ball?

Painter: Yeah.

Me: Have you ever picked up an orange?

Painter: Yeah.

Me: Well, an orange is about the same size as a tennis ball. Which one is heavier?

Painter: The orange could be bigger.

Me: Well, we can weigh them this afternoon, on my kitchen scale, or on the scale we made with the board to weigh the weights outside. [Thinking until we get into the parking spot.] Ah... here's one that you can't dispute. Let's say I give you an empty water bottle.

Painter: Okay.

Me: Now I give you another water bottle, filled with water. Do you think they'll weigh the same?

Painter: [laughing] No! I think the water bottle with the water would be a lot heavier.

Me: You're right... see, they're the same size, but the empty water bottle isn't really empty. It's full of air. And water is a lot denser than air. So that's density.

Painter: So... you mean nothing's ever empty? There's always air?

Me: Yeah, nothing on the earth is ever really empty. Well, unless it's in a vacuum... [Very glad that we're almost to the classroom now.]

Fun. But I wonder how soon he'll outgrow my explanations?