system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do a sick person's eyes look "glassy"?

What causes the change in appearance?
gpt
Here are three reasons - tears, fatigue, and low blood sugar.

Extra tearing caused by a ramped up immune system makes the eyes look more "shiny". 

Fatigue from fighting the sickness make people less likely to focus or be as alert, which make the eyes look "dull". 

Finally, hypoglycemia can give the eyes a glassy look too; being feverish contributes to having lower blood sugar. 

If someone's eyes look glassy, it's a good indication they're unwell because it means they're fatigued and their immune system is running high, making the eyes more watery.