"Cold sore" is a common name for a mouth sore caused by herpes, usually HSV-1. You most often see cold sores on the outside of your mouth, around the lips. Some people also get cold sores inside, on the roof of the mouth, the tongue, and gums. They look different from a canker sore and usually erupt in clusters of small blisters filled with fluid. Some people call these fever blisters.