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Answer by Crys
In you trachea just before you enter the lungs is a thing called the carina. Its like the cough reflex. When you have a cold you have mucus... it tries to drain into your lungs and hits the carina that in turn makes you cough. In general when you cough it can be from something going down the wrong pipe (trachea instead of esophagus) and hitting the carina. It helps keep things that don't belong in the lungs out of the lungs.