Canker sores are small shallow and painful lesions inside the mouth. Locally, it is known as singaw. It's not contagious but it can make eating and talking very painful.
What causes canker sore? It usually starts with a small injury (from teeth braces, accidental biting and abrasion from the teeth). In some cases, it could be stress-related or may be due to a lack of vitamin Bs.
If you have this problem like me, I'm sure you've tried a lot of home remedies that doesn't really work. Don't try putting calamansi, salt or what have you.