Why do stars twinkle?

Between you, the observer on the earth and the distant stars, is an incredibly large distance. When you consider how many dust particles and veil of clouds can be at that distance between you and the stars, it is easy to imagine that the light from the star is sometimes interrupted for a moment.

Once a dust particle or something similar shifts between you and the star, the light is dimmed a bit and then immediately it comes back brighter. Switching between the turbid and the bright light makes us see the stars twinkling.
2012-01-23 at 15:25

Another reason may lie in different air layers of varied warm temperature, breaking the light in different ways. Since the air is constantly changing because of the wind, there is a change of different atmospheric layers, that can bring the stars to twinkle.

That air of different heat, breaks the light differently, you can also see at a hot summer day, when the air shimmers on the streets.
2012-01-30 at 23:35