Aaaaachooo! Sneezing, also known as sternutation, is the rapid and involuntary expulsion of air from the lungs through the nose and mouth. When you inhale before a sneeze, the pressure in your chest increases. Then, when you exhale during the act, the pressure drops.
This causes a fleeting change to the heart rate, which quickly corrects itself. So, although you may feel like your heart has skipped a beat, the trusty muscle does not stop beating.
Sneezing can be good for you because it helps rid the body of respiratory irritants, such as pollen and dust. However, suppressing a sneeze, by holding your breath or pinching your nose, might even be bad for you.
One computer simulation found that, compared with a normal sneeze, it can raise the pressure inside the airways by up to 24 times, resulting in possible damage. Injuries can range from relatively minor, like a ruptured eardrum or burst blood vessel in the nose, to the life-threatening, such as the rupturing of a brain aneurysm, or the trapping of air in the diaphragm leading to the collapse of the lungs.
Remember: sneeze, but sneeze with care. The activity can propel droplets and germs at speeds of up to 160km/h (100mph), so always cover your nose and mouth!