why do stars appear to twinkle????
You can also refer to the answer posted by @Vaibhav, he's written well.
A couple of points...
Stars twinkle because our atmosphere is constantly moving around or is turbulent. The light travelling from the stars gets bent in different direction making them twinkle. It is quite similar to the effect when viewing the reflection of the moon on a lake's surface when its (lake) moving.
@Vaibhav, Good answer, keep it going!
Stars twinkle when we see them from the Earth's surface because we are viewing them through thick layers of turbulent (moving) air in the Earth's atmosphere.