Question

why do stars twinkle?

Answer 1

the appear position of star is slightly different from the actual position due to refraction of star light by the atmosphere 
further this apparent position is not stationary but keeps on changing due to change in atmospheric conditions like density,temperature etc
the light rays coming from stars go on varying slightly
the amount of light entering our eyes form a particular star increases  or decreases randomly with time

Answer 2

Stars twinkle because of turbulence in the atmosphere of the Earth. As the atmosphere churns, the light from the star is refracted in different directions. This causes the star's image to change slightly in brightness and position, hence "twinkle." This is one of the reasons the Hubble telescope is so successful: in space, there is no atmosphere to make the stars twinkle, allowing a much better image to be obtained.