Other people covered it, but I happen to have a Fischer Space Pen on my desk right now so I'll take a shot. You don't want a pencil because bits can break off a

Other people covered it, but I happen to have a Fischer Space Pen on my desk right now so I'll take a shot. You don't want a pencil because bits can break off and make a mess, so you want a pen. The story with the Space Pen is that it has a pressurized cartridge so it works in space (I can't confirm whether other types of pens don't work). Somewhat off topic, the Space Pen was researched 100% with private funds and both the US and USSR space programs bought them retail, in contrast to some of the stories floating around the internet.