On cold winter mornings, the inside of my bedroom window is often covered with tiny drops of water. This report explains what causes them. The first cause is the moisture in the air of my room. Air always holds some water vapor, and breathing, cooking, and drying laundry indoors add even more. The second cause is the temperature of the glass. Because the window faces the freezing air outside, the glass becomes much colder than the rest of the room. When warm, moist air touches this cold surface, the air cools quickly and cannot hold all of its vapor. As a result, some of the vapor turns into liquid water on the glass. The third cause is the lack of fresh air at night. With the door and the window closed for hours, the moisture has nowhere to go, so the drops grow larger by morning. Opening the window briefly each morning lets damp air escape and keeps the glass much clearer.