Blinking is the rapi closing and opening of the eyelid. It is an essential function of the eye that helps spread tears across and remove irritants from the surface of the cornea and conjunctiva. On average, a blink takes approximately 300 to 400 milliseconds. Blink speed can be affected by elements such as fatigue, eye injury, medication, and disease. A person approximately blinks once every two to ten seconds. The blinking rate is determined by the "blinking center", but it can also be affected by external stimulus. When an animal (usually human) chooses to blink only one eye as a signal to another in a social setting (a form of body language), it is known as winking. However, some animals (for example, tortoises and hamsters) blink their eyes independently of each other.