Even a simple sip of water can feel painful within your mouth if you have canker sores. A canker sore shows up as a yellowish white lesion round or an oval lesion with a red border. Canker sores can pop out on the inside of your cheeks or your tongue or gums. Apart from stress acidic fruits braces and dentures can irritate your mouth to cause canker sores. While canker sores go away on their own in a few days these home remedies can make them tolerable. Placing a wet tea bag on the sore can help provide some relief.