system
Given the following post, respond as if you're commenting on Reddit:
human
Looking at stars on a clear night, why do they seem to be gently flickering?
gpt
It’s because of the atmosphere.

The atmosphere isn’t at a uniform temperature.  There are warmer and cooler pockets of air, as well as a temperature change as you increase in altitude.   All this uneven temperature causes the air to sort of mix and undulate a bit.  Hang gliders use that undulating air to stay aloft, but the downside is that it can mess with stargazing.   If you’ve ever seen that shimmering “mirage” on a very hot day on a road in the distance, what’s caused by the hot air rising off the asphalt, it’s the same phenomenon you’re seeing when stargazing: different temperatures of air moving about.   Those different temperatures (which results in varying air density) affects the direction of rays of light going through it, like starlight.   And so, as the warm air rises off the earth, you see a twinkle.

Fun fact:  why did they make the Hubble Space Telescope?  No atmosphere = no twinkling!