How does a pen work?

A tiny rolling ball, a slit thinner than a hair, and ink that climbs by itself. At the tip sits a tiny steel or tungsten-carbide ball, 0.5 to 1 mm across, held by a brass socket so it can spin but not fall out.

Read how it works: How does a pen work? · The history of the pen · More boxes on Glassbox

Chapters in this interactive model

1. Inside a ballpoint pen: A tube of thick ink, a tiny ball, a spring, and a clever button.
2. The rolling ball: It picks up ink on top and lays it down underneath.
3. Why a ballpoint needs gravity: Point it at the ceiling and the ink walks away from the ball.
4. Inside a fountain pen: Ink runs down a slit; air bubbles up to take its place.
5. Capillary action: Why ink climbs into narrow gaps all by itself.
6. How the click works: A tiny rotating cam that remembers whether the pen is out or in.