Why does an oyster make a pearl?

Evidently the pearl formation is in response to irritation within the oyster shell caused by some piece of grit that got in there. If the foreign body gets firmly lodged rather than getting flushed out, the body of the oyster begins to put out a gooey substance which surrounds it to prevent further irritation to the soft oyster. Over time it solidifies into a small round ball, and, voila! one pearl.