Canker sores are small, painful ulcers that develop inside the mouth, often on the inner cheeks, gums, or tongue. They usually appear as round or oval lesions with a white or yellow center and a red halo. The exact cause of canker sores is not fully understood, but they can be triggered by factors such as stress, certain foods, or minor injuries to the mouth. These sores are not contagious and typically heal on their own within one to two weeks. Over-the-counter treatments can help relieve pain and discomfort, while maintaining good oral hygiene can prevent further irritation.