Canker sores are painful blisters that form inside of the mouth. These blisters can form on the tongue, throat, inner cheeks and gums in varying sizes. The exact reason for the formation of canker sores is unknown, however, there are a few different things that are known to attribute to the formation of the sores. Simple sores are formed when the tissue inside of the mouth is irritated or the body has been under a great deal of stress. Certain acidic foods may also cause the sores, but are more likely to irritate them. These foods include many different fruits with a sour taste, such as lemons. More complex canker sores may occur when a person has an immune system disorder, nutritional imbalances and certain diseases.