That small bump that sometimes appears on either the upper or lower eyelid is caused by a blocked oil gland. A “chalazion” usually disappears in about a month without treatment. However, most patients seek treatment when a chalazion is large enough to block vision or looks unsightly. The bump that characterizes a chalazion is caused by a blockage in the Meibomian gland on the eyelid, which produces oil in both the upper and lower eyelids. Chalazia are sometimes confused with styes, which, unlike chalazia, are caused by infections and are usually painful. In addition, chalazia are usually found away from the edge of the eyelid while styes are most often found right on the eyelid edge.
P.S. Home care for a chalazia includes a warm compress to the eyelid several times a day for about ten minutes at a time. If the chalazion does not heal after treatment, it can be surgically removed.