Fountain pens rely on capillary action to draw ink steadily down to the writing nib.

The longer version

Narrow channels draw liquid along themselves without any pump, because attraction between the liquid and the channel walls outweighs the liquid's own weight. A fountain pen uses that: a thin slit in the nib and a feed with fine fins carry ink steadily to the tip, while air travels back up the same path to replace it.

The same effect wicks water along a paper towel and moves moisture through plants. Getting the exchange right is why a pen sometimes leaks when the air path is blocked or the ink is too thin, and why a pen writes poorly once dried ink narrows the channels. Gravity helps, but the channels do most of the work.

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.