There’s an old joke told to illustrate the difference between American and Russian societies, that goes that when NASA needed a writing utensil that worked in space, they spent millions of dollars in taxpayer money to create the Space Pen. The Soviets just used a pencil.
While this is all well and good and har har har, Chris Higgins over at mental_floss actually looks into what happened, and it turns out it’s not so simple:
The final mark against pencils has to do with fire. Any flammable material in a high-oxygen environment is a hazard, as we all learned after the terrible fire on Apollo 1. After that tragedy, NASA sought to minimize the use of flammable materials in space capsules—and every form of pencil (traditional, mechanical, or grease) involved some amount of flammable material, even if it was just the graphite.
And this is why I only use watercolors to write anything. Not as flammable, but OH SO PRETTY.