Dear Humans,

We made posters of Earth’s atmosphere yesterday. And you were very careful to separate it up into its five layers–troposphere, stratosphere, mesosphere, thermosphere, and exosphere. Because that’s what the assignment told you to do.

And you drew the lines between the atmospheric layers in thick, black marker. Because that’s what we humans like to do. We like to draw big, bold lines between things. The lines between states or countries. The lines between eras of history.

But most of these lines are actually pretty squiggly and squishy. That doesn’t mean they aren’t real: they’re based on real factors such as temperature and air density. It just means that the lines are drawn by human hands and human minds. The atmosphere didn’t decide to put itself into five layers. We studied the atmosphere and decided that was a good way to make sense of a really complex thing.

Because that’s what humans do.

Sincerely,

Mr. Curt