Boogers. Snot. Phlegm. Whatever you call it, there's a good reason it exists: mucus. Mucus is a stringy, sticky fluid that coats the inside of your nose, mouth, sinuses, throat, lungs and gastrointestinal tract -- and it's there all the time, not just when you have a cold. In fact, your body makes between about one-quarter and half a gallon (1 and 2 liters, roughly) of mucus every day; and that includes the new batch your nose whips up every 20 minutes [source: Watson].
Mucus acts as a lubricant, and without it, body tissues would dry out. Because it's sticky, it also acts as a trash collector, keeping bacteria and debris (such as pollen, dirt, fungi, smoke or whatever else you might breathe in) from invading your body as you breathe.