Saliva moves from a salivary gland via tiny pathways. A mucocele forms when one of these pathways gets blocked or otherwise damaged, whereby forming a cyst. This lump is usually firm to the touch and may change in size.

Mucocele cysts are usually found inside the lower lip, on the roof of the mouth, inside the cheek, or on the floor of the mouth. Frequently they are formed when one sustains an injury to the face, and are also common if one repeatedly bites or sucks on the lower lip.