Why stars twinkle at night?

The twinkling of stars is merely an optical illusion. In reality, stars do not twinkle, they just seem to be twinkling, because of atmospheric refraction.

Stars are visible to us through several layers of the atmosphere that vary in density and temperature, due to this turbulence in the atmosphere, the light coming from the stars undergoes refraction in a continuous manner until it reaches the Earth. Sometimes they appear brighter and sometimes they appear fainter. Thus, they seem to be twinkling.