The airplane was approaching its destination, at 10,000 feet, and the seat belt sign had been illuminated for 10-15 minutes, when the crew noticed that clouds ahead "appeared bumpy." The flight attendant was notified, and she advised the passengers of possible turbulence via the public address system. A passenger subsequently got out of her seat to use the lavatory, and upon leaving the lavatory, while the airplane was in a cloud and experiencing turbulence, she fell and broke her ankle.