"Twinkle, twinkle little star"

[test5]

Why do stars appear to twinkle?

[EllieD]

The stars themselves do not twinkle, but our eyes perceive that they do due to fluctuations in the density of the atmosphere.

[timshadie]

Its like when you see a heat shimmer or hays off a hot surface things apear to move the same affect happens through the atmosphere and the light from the star apears to twinkle.

[slimfandango]

Some of the twinkle is due to the way your eyes work.
Check out a paper by Mark Mon-Williams, which explains it all.