What is a one-sentence summary of the following article?
Although you might not be aware of it, your body is making mucus all the time, sometimes as much as a quart per day. Even when you are feeling perfectly fine, cells in your nose and mouth called “goblet cells” are combining water, proteins, and polysaccharides into mucus, forming its characteristic sticky texture.  There is a very important reason for this: because mucus is sticky, it is able to trap irritating or dangerous particles before they reach your lungs.  Without mucus, the particles of dust and dirt that you may see when you blow your nose would end up inside your body. When you are sick, your body produces more mucus to ward off the invader, be it a virus or bacteria.  This is why you often only notice mucus when you are sick. Under normal circumstances, you are able to swallow mucus at the same pace that your body produces it, but under adverse circumstances, mucus is being produced faster and in greater quantities, leading the excess to clog your nose. When mucus mixes with saliva and white blood cells, it becomes phlegm. Mucus production can also be stimulated by food, environmental factors, allergens, cigarette smoke, chemicals, and perfumes. When this increased production happens, your sinuses can get blocked, leading to the buildup of bacteria and possibly a sinus infection. Many people believe that the color of your mucus reveals the kind of affliction you are dealing with. While there is some usefulness in these guides, doctors do not really use them to make diagnoses or prescribe treatments.  Generally, healthy mucus should be clear. If your mucus is cloudy or white, you may have a cold. Yellow or green mucus may signal a bacterial infection. If you’re trying to figure out whether you have a cold or a sinus infection, a better gauge is how long your symptoms last. With a cold, you will usually have a runny nose followed by a stuffy nose, each lasting for two or three days. Sinus infections can linger for a week or more.

Summary:
Thank mucus for keeping your lungs clear. Notice your body’s response. Don’t put too much faith in color.