Final answer:
Sand stuck between a bivalve's mantle and shell can lead to pearl formation, as layers of calcium carbonate are secreted around the irritant.
Step-by-step explanation:
When sand gets stuck between the mantle and the shell of a bivalve, it can secrete layers of calcium carbonate to make a pearl. Bivalves like oysters and mussels can form pearls around foreign particles as a defense mechanism. This process, where the mantle secretes a calcareous nacre known as "mother of pearl," not only leads to the creation of pearls but also protects the bivalve from potential irritants.