The sky is the vast space you see when you look up from Earth's surface. During the day, it appears blue because of how sunlight scatters through our atmosphere, the thick blanket of air surrounding our planet. At night, when your part of Earth faces away from the sun, you see the darkness of space dotted with stars, planets, and sometimes the moon.

What we call the sky isn't really a solid thing or a specific place. It's our view through Earth's atmosphere into space beyond. On a clear day, you're looking through about 60 miles of air. When you see clouds drifting across the sky, you're watching water vapor floating in that atmosphere. The sky looks different depending on weather, time of day, and even how much pollution is in the air.

People often use sky metaphorically to suggest limitless possibility, like when someone says “the sky's the limit” to mean there are no boundaries to what you can achieve. Something might be praised to the skies, meaning enthusiastically and without reservation.