Q169: On a clear, dark night, our eyes can see about 6,000 or so stars in the sky. They seem to twinkle, or change their brightness, all the time. In fact, most of the stars are shining with a steady light. The movement of air (sometimes called turbulence) in the atmosphere of Earth causes the starlight to get slightly bent as it travels from the distant star through the atmosphere down to us on the ground. This means that some of the light reaches us directly and some gets bent slightly away. To our eyes, this makes the star seem to twinkle.
You will notice that stars closer to the horizon will appear to twinkle more than other stars. This is because there is a lot more atmosphere between you and a star near the horizon than between you and a star higher in the sky. Go out some night soon and have a look!