A sneeze is a reaction in the body that suddenly presses air out of lungs so it flows quickly out through the mouth and nose. Air and mucus (the liquid found in the nose) are forced out quickly. Sneezing often happens after breathing dust or other small things that irritate the membranes in the nose. The function of sneezing is to clean the space in the nose by removing the mucus and irritating things. Sneezing is mostly involuntary, which means that it happens without trying. 