why do sinuses get stuffy?

Someone ask: why do sinuses get stuffy?

Sinuses get stuffy when lining inside gets swollen and makes thick mucus. Germ, dust, smoke, or bad air can make body sound alarm, so blood rush in and tissue puff up like wet sponge. Then tiny drain holes get too small, mucus cannot flow out well, and pressure build. That trapped goo makes head feel full and nose blocked. Sometimes virus starts it, sometimes allergies, sometimes dry air. Caveman say sinuses are like little caves with narrow exits: if walls swell, cave fills with slime and air cannot move right. Cave wisdom: stuffy sinus mean drain path got squeezed, not that nose forgot how to breathe.