Why can you see your breath on a cold mirror?

When you exhale on a cold mirror, the water vapor in your breath condenses into tiny liquid droplets. These droplets are so small that they scatter light, which is why you see a cloudy mist. As the droplets grow larger, they fall down the mirror, leaving behind a clear patch.

The colder the mirror, the smaller the droplets will be and the cloudier the mist will appear. This is because there is less energy available to the water molecules to evaporate, so they stay closer together in liquid form.

The opposite is true when you breathe on a hot mirror. The heat from the mirror causes the water molecules to evaporate, so you don't see any mist.