system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5 Where does all the fluid come from in a runny nose? How is there so much?
gpt
A runny nose is generally the product of the body making mucus in an effort to expel irritating substances or infectious agents. Think dust, pollen, germs, etc. This mucus or snot is composed of a small amount of protein and a large amount of water.

The water is sourced from your blood. This is one of the main reasons why you need to drink plenty of fluids when you are sick, as you are expelling fluids through mucus. 90-95% of snot is water so the amount of proteins your mucus glands need to supply is low compared to the total volume which is why it seems so endless.