The secret behind the pens that we sell is the amazing Fisher Space Pen refill. It is a pressurized cartridge that allows ink to flow instantly and dependably no matter what. It allows you to write under water, upside down, on greasy or dirty surfaces, or in boiling or freezing temperatures (the waterproof ink flows between -30 F (-35 C) and 250 F (121 C). Pressurized at almost 35 pounds per square inch, the visco-elastic ink is delivered to the tungsten carbide ball point without leaking or globs. These refills fit any pen sold on this website.