You may have heard the old joke: If your nose is running and your feet smell, you must be upside down! But why does your nose run? Read on to find out the whole story.

What's Running?

To understand why your nose runs, you need to know what mucus (say: MYOO-kus) is. This is the gooey, sticky, slimy material that's made inside your nose (also known as snot). Believe it or not, your nose and sinuses make about a quart of snot every day!

For something kind of gross, mucus does a lot of good. It keeps germs, dirt, pollen, and bacteria from getting into your lungs by stopping them in your nose. But sometimes mucus doesn't stay put.

Reasons for Running

If your nose is running, there are several possible explanations:

You have acoldor theflu: When you have either one of these, your nose goes into mucus-making overdrive to keep the germ invaders out of your lungs and the rest of your body, where they might make you even sicker than you already are. You know what happens then: The mucus runs down your throat, out your nose, or into a tissue when you blow your nose. Or it can fill your sinuses, which is why you get that stuffy feeling.

You haveallergies: Kids who have allergies get runny noses when they're around the thing they're allergic to (like pollen or animal hair). That's because their bodies react to these things like they're germs.

You're crying: When you cry, tears come out of the tear glands under your eyelids and drain through the tear ducts that empty into your nose. Tears mix with mucus there and your nose runs.

Baby, it's cold outside: When you're outside on a cold day, your nose tries its best to warm up the cold air you breathe before sending it to the lungs. Tiny blood vessels inside your nostrils open wider (dilate), helping to warm up that air. But that extra blood flow leads to more mucus production. You know what happens next. Drip, drip, drip.

Stoppin' the Runnin'

If you have allergies, your doctor might give you medicine called an antihistamine (say: an-tye-HISS-tuh-meen). But sometimes the easiest thing to do is — you guessed it — blow your nose!