There are many reasons your nose might run.
If you have a cold or the flu, 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. Hairs in your airways called cilia (say: sill-ee-ah) also work to get bacteria and dirt out of your lungs and into your mucus. Then the mucus runs down your throat, out your nose, or into a tissue when you blow your nose. Or it can run into your head – which is why you get that stuffy feeling.
Kids who have allergies also 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.
Sometimes your nose will get runny when the air you are breathing is too dry (like when the heat’s on or if you live in a dry, desert climate). You might wake up in the morning with a runny nose because your body’s trying to make up for the fact that the air’s drying out its mucus supply.
You may also notice that your nose runs when you play outside on a cold day. Tiny blood vessels inside your nostrils open wider (dilate), helping to warm up that cold air you’re breathing. But that extra blood flow leads to more mucus production. Then since the air in your nose is a lot warmer than the air around you the air condenses into droplets, just like when you take a shower – the bathroom gets all steamy from the warmth – or when you have cold drink on a hot day and the glass gets covered in water drops. The same kind of thing happens in your nose – water drops come together or condense, mix with your mucus, and run out your nose.
Your nose runs when you cry because when tears come out of your tear ducts in your eyelids, they don’t just run out of your eyes. Behind your nose and eyes is a big open space called the nasal cavity. The tears run down the nasal cavity and out into your nose, where they mix with the mucus there.
For more on burps check “Why Does My Nose Run?” on Kidshealth.org
Answer provided by KidsHealth.org