There are many causes of blisters (also called vesicles or bullae), the small patches of separated skin layers, with fluid between them.
Simple friction is the most common, such as from wearing poorly fitted shoes.
Blisters can also be a skin reaction to excessive heat, either from the sun or from a concentrated heat source. Certain chemical exposure, such as to detergents and solvents, can cause blistering of the skin. The skin may have an allergic reaction to an insect bite.
Some common medical conditions are also responsible, such as chickenpox; cold sores; herpes; impetigo; pompholyx; scabies; and hand, foot and mouth disease (a viral infection that usually affects young children).
Among other, rarer causes that affect children are chronic bullous dermatosis and bullous ichthyosiform erythroderma.