Why do stars twinkle?

We've all heard the nursery rhyme, "Twinkle, Twinkle Little Star" but how do stars twinkle? Just before it reaches your eyes, the starlight that has crossed the cosmos has to travel through a few hundred kilometres of the earth's turbulent atmosphere. As the air moves around the light is refracted, or bent causing the apparent position of the star to 'wobble' slightly. This is why astronomers go to such effort and costs to put their telescopes in orbit, to avoid distortion because of the Earth's atmosphere.

This is a image of the Crab Nebula (M1) taken from Earth