Fog is a thick cloud of tiny water droplets that forms near the ground, making it hard to see very far. When you walk through fog, you might only be able to see a few feet ahead, and distant objects seem to disappear into a gray or white blur.

Fog forms when air near the ground cools down enough that the water vapor in it condenses into millions of microscopic droplets, creating what looks like a cloud you can walk through. Morning fog often appears in valleys or near bodies of water, then burns off as the sun warms the air. Dense fog can be dangerous because drivers can't see the road ahead clearly, which is why cars use special fog lights and ships use foghorns to warn others of their presence.

To fog means to cover something with fog or make it hard to see, like when a bathroom mirror fogs up after a hot shower. The word can also describe mental confusion or forgetfulness. When you're tired and can't think clearly, you might say your mind feels foggy. If you can't quite remember something, it might seem lost in the fog of your memory. After staying up too late, you might feel like you're moving through the next day in a fog, struggling to focus on what people are saying or what you're supposed to be doing.