A blister is a small pocket of fluid that forms between the upper layers of the skin. It usually occurs as a result of friction, burns, or irritation, and can be filled with clear fluid, blood, or pus. Blisters serve as a protective barrier, allowing the underlying skin to heal while preventing infection. Common causes of blisters include wearing ill-fitting shoes, sunburn, or allergic reactions. While most blisters heal on their own, it is important to avoid popping them, as this can lead to infection. Proper care, such as keeping the area clean and covered, can help promote healing.