Why do a clothespin's jaws close when released?

A clothespin can keep holding a sock after the hand that opened it has gone away. The secret is the small metal spring near its middle. The spring keeps pushing the two gripping ends, called jaws, toward each other.

The two long halves can turn near the spring. Each half has a jaw at one end and a handle at the other. Squeezing the handles together makes the jaws spread apart. The ends move in opposite ways because they are on opposite sides of the turning place.

Opening the jaws also bends the spring away from its resting shape. The hand supplies energy to do this. When the handles are released, the spring moves the halves back. The handles spread out as the jaws come together again.

With a sock between them, the jaws cannot close all the way. The spring still presses them inward, so they grip the cloth. That pressure helps the cloth resist slipping. The little spring lets a brief squeeze become a lasting hold.