Where does snot come from?

The nasal cavities are lined with a mucous membrane. It contains cells that continuously make mucus and secrete it into the nasal cavity. The purpose of this is to trap dust particles and microbes that stick to it. This prevents dust and microbes from getting into the trachea. The trachea also contains mucous cells that do the same. This way, no dirt gets into the lungs, because that would cause problems for our breathing.

Slime + dust = snot. This either comes out through the nostrils or is swallowed and then broken down in the stomach.

The mucus that we make consists mainly of water, but also contains salts, mucins (which makes it sticky) and substances with a disinfecting effect (against bacteria and viruses, namely lysozyme and antibodies).