Today’s Rhymes With Orange, with a subtle pun on cavity:
The cavity entry in NOAD2:
an empty space within a solid object, in particular the human body: the abdominal cavity | a body cavity.
• a decayed part of a tooth.
The second, bulleted, sense is a specialized sense of the first: a cavity in a tooth is indeed an empty space within the tooth, but it’s not a part of the tooth in the way that the nasal cavity is a part (an internal part) of the nose; instead, it’s a defect in the tooth.