The eyelid is a thin fold of skin that covers and protects the eye. It helps keep the eye moist by spreading tears across the surface and prevents foreign objects from entering. Eyelids also play a role in regulating light exposure, closing during bright conditions or when sleeping. There are two main eyelids: the upper eyelid and the lower eyelid. The upper eyelid is usually larger and more mobile, allowing for blinking and closing. Eyelids contain muscles, such as the orbicularis oculi, which control their movement, and are lined with sensitive skin that can react to irritants.