Why Number 2 Pencils Are Used on Exams

Host Michael Aranda explains why number 2 pencils are used when scoring exams in a recent episode of the SciShow series Quick Questions. Essentially, the automated machines that score exams (the kind that require the filling in of bubbles that should be familiar to any current or former student) function by reading where certain graphite marks are on the sheet, and number 2 (or HB) pencils are the exact amount of hardness that gives a good reading when filled but doesn’t provide false positives when thoroughly erased.