Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Canker Sores ?
    ### Response:
    Canker sores are small, round sores in your mouth. They can be on the inside of your cheek, under your tongue, or in the back of your throat. They usually have a red edge and a gray center. They can be quite painful. They are not the same as cold sores, which are caused by herpes simplex.
    