The States of Matter

Tonight we were reading In the Night Kitchen, in which the main character ends up swimming in milk, which led to discussions of liquid: "Milk is a liquid. Water and juice are liquids. PEE IS A LIQUID!" Lucy then began talking about solids: "Duck is a solid. This book is a solid. Mom, you are a solid, even though I can't pick you up." And I said, "If something is not a solid or a liquid, what is it?" She made her thinking face, frowning and squinching her way to..."I don't know..." I prompted, "How about a gas?" She lit up: "Yes, a gas. A gas, a gas is...
(more frowning and squinching)
(then the big, existential part)
...something that is all around you, but you can't see it or feel it. It's there even though you don't know it's there."
Yes. It's there even though I don't know it's there.