Julie Derr, age 9, of Visalia, California, for her question:

Why do we see our breath on cold days?

What you really see are mini clouds of misty moisture. They appear on winter days because they cannot form without a lot of chilly air. The material to make them is the air you breathe out. This comes from deep down inside your lungs. You may not know this, but your body keeps everything inside its skin damp or downright wet. The air you breathe into your spongy lungs gets warm and moist. When you breathe out, it is mixed with molecules of gaseous water vapor.

When the weather is cool or fairly warm, this vapor stays the same. When we breathe it out we cannot see it    because it is an invisible gas. But chilly air does something to this water vapor. It makes molecules of the gas join together and form mini droplets of liquid water. This is the misty moisture that we see on a cold day, when the chilly air changes the moisture from our lungs into puffy little clouds.