When inverted, ballpoints will deposit the ink that's clinging near the tip. But soon, the ink responds to gravity's pull - it flows back down the barrel and the pen no longer writes. Factors such as the stickiness of the ink, the amount of ink in the barrel, the width of the barrel and the width of the ballpoint affect writing time. Roller balls and felt-tip pens rely less on gravity. Inside the barrel of these pens is a fibrous core.
Capillary action pulls the thin ink through the fibers.
In orbit, the downward pull of gravity is not exerted. Pen inks that rely on gravity won't flow, no matter what position they're in. Today, all U.S. astronauts use the failsafe Fisher Space Pen. This activity is a good chance to discuss scientific testing. By making each line last for three seconds, we controlled the variable of time so we could evaluate the pens in terms of number of lines drawn.