Canker sores are painful ulcers that erupt inside the mouth on the gums, cheeks, tongue or lips. Stress, fatigue and food allergies may increase the likelihood of canker sores in some individuals. An injury to the inside of the mouth from a stray toothbrush bristle or a burn from hot liquid or food can also instigate a canker sore. Certain foods often trigger canker sores, such as citrus fruit, vinegar, chocolate and tomatoes. Salt is a natural disinfectant that kills germs and bacteria and promotes fast healing of canker sores.