In weightless situations, the ink inside a classic biros does not flow properly, making writing impossible.
To OVERCOME this problem, Paul Fisher (Fisher Pen Company) invented the Space Pen Anti-Gravity 7 (#AG7) in 1966, first carried into space on board Apollo 7 in 1968, the biros capable of writing in zero gravity.
The innovative aspect of this invention involved the use of a sealed and pressurised ink cartridge that allowed the Space Pen to write in any position and on all types of surfaces. Fisher Pen called this ink 'thixotropic' in that once shaken, it goes from a solid to a liquid state, distributing itself evenly over any surface.