system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What are canker sores and why do they only occur in the mouth?
gpt
Canker sores are small, shallow ulcers that appear in the mouth and often make eating and talking uncomfortable. There are two types of canker sores:

Simple canker sores. These may appear three or four times a year and last up to a week. They typically occur in people ages 10 to 20.
Complex canker sores. These are less common and occur more often in people who have previously had them.
What Causes Canker Sores?
The exact cause of most canker sores is unknown. Stress or tissue injury is thought to be the cause of simple canker sores. Certain foods -- including citrus or acidic fruits and vegetables (such as lemons, oranges, pineapples, apples, figs, tomatoes, and strawberries) -- can trigger a canker sore or make the problem worse. Sometimes a sharp tooth surface or dental appliance, such as braces or ill-fitting dentures, might also trigger canker sores.

Some cases of complex canker sores are caused by an underlying health condition, such as an impaired immune system; nutritional problems, such as vitamin B-12, zinc, folic acid, or iron deficiency; or gastrointestinal tract disease, such as celiac disease or Crohn's disease.


Are Cold Sores and Canker Sores the Same Thing?
No. Although cold sores and canker sores are often confused with each other, they are not the same. Cold sores, also called fever blisters or herpes simplex type 1, are groups of painful, fluid-filled blisters. Unlike canker sores, cold sores are caused by a virus and are extremely contagious. Also, cold sores typically appear outside the mouth -- usually under the nose, around the lips, or under the chin -- while canker sores occur inside the mouth.