Molecules are in everything.
I held up a sweater and a glass bottle, and explained that in the bottle molecules are more closely packed together, making for more density and heaviness than in the sweater where they are further apart. To illustrate this, I drew circles close together and further apart on the board. Every substance in reality, I have learned, is some relation of matter and space, heaviness and lightness, and these are crucial opposites every child and every teacher wants to make sense of. A number of hands shot up. The others agreed.