Fog is really a cloud near the ground. Both fog and clouds are made of many little drops of water. These drops stay in the air because they are so small. You cannot see each drop,but fog can make it hard to see other things. It can be dangerous if you are driving,for example. Sometimes where is a lot of fog you connot see the road. Sailors also have trouble when there is fog. Boats may get lost and hit rocks or beaches in the fog.