system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What makes it foggy in the morning? Need answer for my actual 5 yr old.

I couldn't provide a very good answer for my daughter this morning. Any help is appreciated.
gpt
Basically, "fog" is what happens when there's more moisture in the air than the air can contain while remaining transparent, i.e., there's enough moisture in the air to push the humidity level beyond 100%. That's not possible, so the excess water appears as fog or condenses as dew/frost.

The reason there's fog in the morning which commonly "burns off" by mid-day is that cold air can hold less moisture than warm air, i.e., it will take less water to reach 100% humidity. So given a fixed amount of water in the air, you'll start to see dew and even fog as the temperature drops, both of which will go away as the temperature rises.