system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why I can't use a mechanical pencil on the ACT/SAT.

Also, why can't I just work at my own pace instead of starting and stopping?
gpt
The pencils are restricted to make sure that the machine that grades the tests can read it.  It's also possible to hide stuff inside a mechanical pencil, which they want to avoid.

The sections are timed so that the exam is standardized between students.  Each student spends the same amount of time on each section.  It also prevents you from spending too much time on the first parts and running out of time on the others.