A 7-year-old girl complains of a neoplasm on her lower lip. The neoplasm appeared 3 months ago and has been slowly growing since then. Objectively on the mucosa of the lower right lip there is a round neoplasm 0.5-1 cm in diameter. It protrudes from the mucosa, has smooth surface, and its clear bluish content is visible through its walls. On palpation it is painless and elastic. Make the clinical diagnosis: