To answer the previous question, several theories can be put forward as to why mechanical pencils cannot be used during these tests. One of the most popular theories is that scantron machines are not able to scan the text if it written with a mechanical pencil. The thing is that older types of scantrons work by blasting paper with light, so the answer can be checked if the light is entirely blocked from coming through the paper. The graphite in No. 2 pencils  is good at blocking the light. Graphites of lighter weights and pen ink don’t block enough light and lead to “false positives.”