We have a thin layer of watery, slimy stuff called mucus on the inside of our nose. This mucus plays an important role of trapping the dirt and germs that are in the air we breathe. When we catch a cold, extra mucus is made to help us get rid of the cold germs. This mucus only makes our head feel stuffy and our nose sometimes run. On a really cold day, our nose runs even when we are not sick. The cold air causes the mucus to come together and form larger drops that makes our nose run.