Sneezes are involuntary. Small particles like pollen or dust float up your nose and irritate your membrane and it triggers a whole series of reactions. Or more technically when an irritant is in contact with your nasal mucosa it triggers the trigeminal nerve, which sends a signal that ends up in the lower part of the brain known as the medulla. Your chest expands in response, your lungs fill with air and then you sneeze everywhere. You actually sneeze around 5,000 droplets of mucus and air at speeds of up to 100mph (160kph).
Your eyelids shut down as part of that reaction, but it's one of science’s big mysteries as to why. We think it is an involuntary response, a reflex, like when your leg jerks after being tapped on the knee. What could be causing it? You may shut your eyelids so that when you sneeze out germs they don’t fall on your eyes. Or maybe your eyes shut because they are one of a series of muscles that tighten during that involuntary response.
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