system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: On most standardized tests, why is it so important that we specifically use a #2 pencil?
gpt
It's a compromise between hard enough to make a mark that's readable by the machine (machines worked by sensing opaqueness so #3 and #4 weren't always dark enough to be readable) vs not smudging when erased (#1 is darker than #2, but is harder to cleanly erase without creating a bigger smudge patch that could also lead to errors).  

In truth, the machines have improved with technology and can read most types of marks very accurately, but since #2 remains in the sweet spot (and keeps people from using ink that's much harder to cleanly erase), they still recommend it.