system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do people get dark circles/bags under their eyes when tired or sick?
gpt
It is fluid buildup under your eyes. Since the skin around our eyes is the thinnest skin in our bodies, it shows fluid retention more than other places. 

There are three main reasons why this might happen:

1. Salt. Osmosis means that fluids in your body travel from low-salt areas to high-salt areas. So if you're crying, those salty tears will cause puffiness. 

2. Sickness. Fluid buildup due to colds or allergies will also cause those areas to bulge with liquid and expose the fine veins beneath.

3. Irritation. Rubbing your eyes or leaving makeup on overnight causes minor injury to the skin there, and fluid rushes in to cushion the area, the same way an injury will swell.