Even when you're healthy, your body is a mucus-making machine, churning out about 1 to 1.5 liters of the stuff every day. Most of that mucus trickles down your throat and you don't even notice it.
However, there are times when you do notice your mucus -- usually not because you're producing more of it, but because its consistency has changed.
"Typically, the mucus changes character. It gets thicker," Johns says. "When it has mass effect you feel it, and when you feel it, you want to get rid of it." Some people just naturally have thicker, stickier mucus than others.
It generally takes a bad cold, allergy, or contact with something irritating to throw your body's mucus production into over-action.
For instance, during an allergic response to an offending trigger, such as pollen or ragweed, mast cells in your body squeeze out a substance called histamine, which triggers sneezing, itching, and nasal stuffiness. The tissue of the mucus membranes starts leaking fluid, and your nose begins to run.
Drinking milk may also make some people produce more mucus. Kao says that's due to gustatory rhinitis, a reflex reaction that's triggered by eating. Gustatory rhinitis is also why your nose runs when you eat hot peppers. Milk proteins cause the same type of response in some people. But although you may feel like you have more phlegm, you're not going to worsen a cold by drinking a glass of milk, Johns says.