Oh well, I'll answer this here now.
Mucus is 95% water, the rest being a random mixture of things, including, but not limited to: random carbohydrates, dead leukocytes (also known as Jehovah's Witnesses) and a polysaccharide called "mucin", which gives mucus its sticky texture (combine a polysaccharide with water, you get sticky shit. Starch is a polysaccharide, and I'm sure you've combined flour and water at a point in your life and found out what a bitch it is to get it off your fingers.)
Mucus ain't limited to the nose, but since we're talking all things runny here, we'll limit it to that. Mucus is part of a two-fold defense to keep foreign bodies from being inhaled, the other part being cilla (sp), also known as your nose hairs. The hope is that whatever's trying to make its way into you will get stuck in the hair or the snot, thus preventing inhalation.
However, when some clever little bastard makes its way past the defenses and gives you a cold, your nose gets somewhat of a guilt trip. It turns the mucus production into overdrive, which is why you get a runny nose...it's just trying to flush whatever got in your body, out..usually it just succeeds in unclogging stoppages, which is always a good alternative. So, chances are if your nose is running, you're sick.
Other times your nose will run: if you're ever in a really dry climate...your body's just making up for all the snot that's drying out. And it runs when you cry because you're literally crying out your nose. Tears will run down the nasal cavity and mix with the mucus, which is why the tear-snot is more runny than normal.
So that's pretty much the skinny on snot. Half of it I looked up, half is off the top of my head, which is really sad. It's a wonder more women aren't sleeping with me.
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