Why do stars twinkle?

Stars twinkle because their light passes through Earth's moving atmosphere before it reaches us. Warm and cool air bend the light by slightly different amounts, so the star's point of light shifts brighter and dimmer very quickly. The star itself is steady; our air makes it sparkle.

Planets usually look calmer because they are near enough to show a tiny face, not just a dot. Their light arrives from many spots at once, and the bends mostly cancel. Astronauts above the atmosphere see stars without the shimmer we notice from the ground.