Why use a number 2 pencil in a scan tron tests?

Answer 1

I've always wondered this, assuming that the scantron machines were simply light v. dark detectors. Turns out, the first scantron scanners were based on reflected light and such, so the #2 graphite was ideal for accurate scoring. Other numbers indicate different hardness, so the mark was either too smudged or not dark enough.

Luckily, technology has advanced, and these days a scantron will basically pick up anything. Most are designed not to pick up any color similar to that of the test paper, so avoid any mistakes, but they should be able to accurately read tests done in and pencil, pen, colored pencil...etc.
So really, the whole #2 pencil hype is outdated.