Bart Owings~ age 8. of ‑Atlanta. Gal.. for his question:

What causes sweat on car windows?

That dewy moisture comes from the air. Warm air is always thirsty. It drinks up water in the form of vapor gas. When the air cools it loses its thirst„ It gives back the vapor in the form of drops of water. The sweat on the car windows comes from this.

Sometimes the windows steam up on the inside. This is when the inside of the oar is very warm warmer than the outdoors. The air drinks up whatever moisture it can find. It evaporates the perspiration from your body and turns it into vapor gas. There is always a little moisture in the air you breathe out. The thirsty air grabs this too.

The glass windows are cooler than the hot inside of the car. The air that touches them is cooled off. When this happens. it sets down some of its vapor in the form of water droplets ‑ it sweats on the cool glass.

The opposite may happen if the inside of the car is cooler than the outdoors. Warm air  full of vapor  touches the cool glass on the outside. It cools off and drops some of its vapor as dewy moisture. The outside of the car windows become covered with sweat.