Blisters are small, fluid-filled sacs that form on the skin's surface, usually as a response to friction, heat, or irritation. They can appear anywhere on the body but are most common on the hands and feet. The fluid inside a blister helps protect the underlying skin as it heals. Blisters can be caused by various factors, including sunburn, burns, or allergic reactions. While most blisters heal on their own, it's important to keep them clean and avoid popping them to prevent infection. If a blister becomes painful or shows signs of infection, medical attention may be necessary.