That’s a great observation. When you breathe out, you let a couple of different things into the air.

Not only do you breathe out carbon dioxide, but you also breathe out teeny tiny droplets of water. These water droplets are so small we can’t see them with our eyes.

Cigdem Capan.

My friend Cigdem Capan, a physics instructor at Washington State University, said one big factor that can help water move between these different states of matter is temperature.

When you breathe on a mirror, you are helping water move from a gas state to a liquid state. The surface of the mirror is a lot colder than the water vapor that comes from your warm human body. If you breathe on a mirror, you can easily feel that heat releasing into the air.

“If you are wearing eyeglasses and you are wearing a face mask, you can also see the glass fog up,” Capan said.

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