Fog
Fog is a cloud that forms close to the ground when warm, moist air cools down fast.
Think of it this way
When a hot pot of soup sits on the counter, steam rises into the cool air above it. The tiny water drops in that steam are too small to see on their own. But when enough drops crowd together, they form a white haze. That haze is just like fog — a cloud of water drops floating low near a surface.
Explaining fog by grade level
You know how your warm breath makes a little cloud on a cold day? Fog works the same way. Warm wet air meets cool air near the ground. The water you can not see turns into tiny drops you can see.