Once sneezing seems to be rare. Because almost always comes a second and often a third sneeze behind. But why? You have to know why we sneeze at all. The reason are foreign bodies in our nose, which we inhaled and want to get rid of before they get into the lungs. Sneezing is often not enough, so the tactic is repeated until the airways are cleared. This can happen with smoke, air pollution, dust, mold or mildew, much more often in case of a cold or allergies.