A sneeze is a reaction in the nose in which air and mucus (the liquid found in the nose) are forced out quickly. It often happens after breathing dust or other small things into the nose. When people sneeze, their eyes close. This is a natural reflex which keeps the eyes from being bulged by the great pressure of air which is being released through the sinus cavity (the area the nostrils lead into).
A common thing to say to people when they sneeze is "Bless you!" or in the United States sometimes "Gesundheit!" (from German, meaning "Good health"). There is a story that the blessing started with Pope Gregory VII (540–604 AD). He said a blessing might stop people from getting the bubonic plague which was killing people in Rome at the time. When people sneeze the speed is about 150 kilometers per hour or more.