Oral mucoceles, sometimes known as oral mucous cysts, are painless blisters that occur on the surface of the mouth. These cysts often appear clear or blue in color while smooth and round in shape. They commonly develop on the lower lip's inner surface, inside the cheeks, and the bottom surface of the tongue. Though mucoceles are usually painless, they may get in the way of chewing, speaking, swallowing, and breathing, depending on their size, which ranges from one millimeter to two centimeters.