system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: does writing on your skin with a ballpen damage the ballpen or the ink inside it? Is there something about the oil on the skin that makes the ballpen stop working?
gpt
A ballpoint pen works by having a tiny ball held such that it blocks the opening to a reservoir of viscous sticky ink. When the ball is rotated by friction against a surface the ink will stick to the ball and be dragged out, applying it to the surface.

If you draw on your skin then your oily skin will apply oil to the ball. This will reduce the friction of the ball against the surface and make it more difficult to turn. The coating of oil will also make it harder for the ink to stick to the ball and be drawn out of the reservoir.

In summary it just makes it not work as well.