Safety pins stay closed because the spring in the bend keeps pressure on the clasped head.

The longer version

The coil at the bend of a safety pin is a spring held under constant tension. It pushes the two arms apart, which presses the pointed end firmly into the clasp and keeps it from working loose. The clasp also covers the sharp tip, so the stored energy in that coil cannot do any harm while the pin is shut.

The device was patented in the nineteenth century and replaced straight pins that had fastened cloth for centuries. Straight pins rely only on friction and work out of fabric; the spring and the shielded head solved both problems at once. Modern versions keep the original shape, formed from a single length of wire.

Where this entry sits

Odd Little Things looks at the ordinary objects people touch every day and the small mechanisms and histories hiding inside them. Buttons, zips, pencils, mirrors, stairs, stamps, packaging, and road signs all get their turn. The full list sits on the Odd Little Things page, where every entry is listed in order.