Why does the ink in a pen only run in one direction?

Question

This has stumped me.

Answer 1

Capillary action.

Practically, it doesn't just run in one direction. Many's the time I've put an almost dry ballpoint pen into a half-sock for a stocking so I could use centrifical force to urge the last of the ink to the pen's tip.

Answer 2

Gravity.

Answer 3

it only runs in one direction unless..... it leaks... Then it will find anything white, pink, important.. in the washer it will run in every direction.