A chalazion is sometimes the aftereffect of a sty. It is less tender but lasts longer. Natural oils from the eyelid's oil glands must drain through ducts out to the eyelashes. If debris blocks this normal drainage, it may cause a sty or chalazion. Sometimes, this debris accumulates because of a condition called blepharitis, a long-standing inflammation of the edges of the eyelids, with redness, thickening and scales and crusts.