system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can you only use a #2 pencil on a scantron sheet?
gpt
It's not really an issue of whether it works, but of who is responsible in the event that it doesn't work.

They aren't going to test their machine with every possible type of pencil.  Even if they tested it with a variety, *someone* would find *some* pencil that doesn't scan right.  So instead, they test it with the most common type of pencil, and if you use something else and your answers aren't scored right, it's your own fault.