Your skin is versatile. It has to be able to protect your body’s interior from outside forces and pathogens, while still remaining flexible enough to stretch and move with you. Still, pressure and friction can cause damage.

Blisters are fluid-filled bubbles that appear in the skin when it has been subjected to an extended period of rubbing. The damaged top layers of skin separate from the lower layers to protect them. The space between then fills with clear serum.

The resulting bump is very sensitive to the touch, which can make it painful to wear shoes or participate in some activities.

These sores develop fairly quickly.

Wearing too-tight or stiff footwear can create one or more in just one day—or even a few hours.

You can develop blisters almost immediately from other sources, too, like second-degree burns from extreme heat. Some first degree burns, chemicals, and allergies may cause vesicles to pop up a few days after the initial exposure. Even some infections can raise these bumps on the skin.

Usually small sores like these aren’t serious, but they can become infected. They are a weak point in the skin where bacteria could potentially enter.

If a blister develops an infection but stays intact, the fluid inside may fill with pus and ooze.