Because your mouth and your nose are connected. [Look at this image](https://media.sciencephoto.com/image/p4960019/800wm/P4960019-Nose,_mouth_and_throat.jpg). You can see that your nose and mouth basically merge into a single cavity at the back of your throat. Since they share a common opening, anything that goes in or comes out of one can go in or come out of the other.
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