Ballpoint and rollerball pens are designed to keep air out from the ink cartridge. The ball in the nib is designed to act as a cap to stop air getting through. However, air can still get inside the pen especially if the pen is left uncapped. More often however, people assume ballpoint pens have dried out because they stop writing. A better explanation would be that the thick ink inside the cartridge cannot flow as freely as needed. Additionally, ballpoint pens can stop working because the ink left in the ball when unused has been exposed to air and as a result clogged. Both issues can be fixed rather easily however so you can get your pen to start working again.
