Why do stars twinkle?

I know iv heard the reasoning before but iv forgotten and I was stumped the other day when someone asked me.

Thank's for asking that mate,i really dont know also the answer that why i seach it,and it says like this:

The scientific name for the twinkling of stars is stellar scintillation (or astronomical scintillation). Stars twinkle when we see them from the Earth's surface because we are viewing them through thick layers of turbulent (moving) air in the Earth's atmosphere.

Stars (except for the Sun) appear as tiny dots in the sky; as their light travels through the many layers of the Earth's atmosphere, the light of the star is bent (refracted) many times and in random directions (light is bent when it hits a change in density - like a pocket of cold air or hot air). This random refraction results in the star winking out (it looks as though the star moves a bit, and our eye interprets this as twinkling).

Stars closer to the horizon appear to twinkle more than stars that are overhead - this is because the light of stars near the horizon has to travel through more air than the light of stars overhead and so is subject to more refraction. Also, planets do not usually twinkle, because they are so close to us; they appear big enough that the twinkling is not noticeable (except when the air is extremely turbulent).

Stars would not appear to twinkle if we viewed them from outer space (or from a planet/moon that didn't have an atmosphere).

Stars appear to twinkle because the light from them must reach us by passing through the atmosphere of the earth. By doing this, the light will experience some "distortion" (refraction is the physics term) to a greater or lesser degree.

We see this because of light's passage through layers of air with differing temperatures, pressures, densities, moisture content and other factors. All of them will have a subtle effect on the light, and it will "waver" a bit so that we get a "twinkle" when we look at the stars. Temperature differences are well known for distorting light, and the "heat waves" we see when objects are viewed through air with differing temperatures in the path of the light are things we can generally recall. On a hot day, air above the heated surface of a dark colored vehicle appears to "shimmer" because of the "heat" rising from the vehicle.

Light from the stars came a long way to get here for us to see it, but it is its passage through earth's atmosphere that gives it the greatest difficulty. And we see the difficulty the light has getting through the atmosphere 'cause it just can't stay in a straight line. The small "shifts" the light takes in its travel appear as the "twinkle" of the stars.

stars only twinkle when we look at them from earth.out in space their light shines steadily.we see them twinkling because of the air around the earth, as light from a star travels towards us,it is bent and wobbled by bubbles of hot and cold air.