Canker sores, also known as aphthous ulcers, are small, painful lesions that develop inside the mouth on the inner cheeks, lips, tongue, or soft palate. They are not contagious and differ from cold sores, which are caused by the herpes virus and occur on the outside of the lips. The exact cause of canker sores is not fully understood, but contributing factors may include minor mouth injuries, stress, hormonal changes, certain foods, vitamin deficiencies, or underlying immune-related conditions. They typically appear as round or oval ulcers with a white or yellow center and a red border and can cause discomfort while eating or speaking. Most canker sores heal on their own within one to two weeks, though topical treatments and oral rinses may help reduce pain and speed healing.