The sky is the expanse of air that surrounds the Earth, appearing as a blue dome during the day. It is made up of gases, primarily nitrogen and oxygen, and contains clouds, which are formed from water vapor. The sky also plays a crucial role in weather patterns and climate. At night, the sky reveals stars, planets, and other celestial bodies, which are part of the universe. The appearance of the sky changes with the time of day and weather conditions, showcasing phenomena like sunsets, rainbows, and auroras.