You probably know the drill: You’re handed an exam, and you’re told you need to fill out the bubbles completely, and then reminded that -- for reasons that no one ever explains -- you can only use a number 2 pencil!   Now, unless you’re an artist, you’ve probably never even seen a pencil that’s not number 2, also known as an HB pencil outside the U.S.    The thing is, if you used any other kind of pencil when you took an automatically-scored test, you’d probably be fine.   However there are reasons that test instructions still tell you to stick to number 2. And they have a lot to do with how machines score your tests -- or at least, how they used to.   In 1931, a high school physics teacher named Reynold Johnson was grading exams when he wondered if there might be a way to score them without actually having to go through every single answer by hand.   He knew that graphite, the writing material in pencils, was electrically conductive. It’s made of flat sheets of carbon, and its outer electrons are relatively free to move around within the material.   So Johnson devised a machine with many small electrical circuits that would pass over an answer sheet. If there was a pencil mark on the sheet, it would conduct electricity through one of the circuits, and the machine could record it as an answer.   Eventually, he sold the idea to IBM, which produced the first mark-sensing machines.   But the number 2 thing didn’t become an issue until the next generation of machines were introduced in the 1960s and 70s.   Those machines had light-sensing devices called phototubes, and if they did not detect light in a certain spot, they recorded it as an answer.   That made graphite especially useful, because it’s better at blocking light than many inks. Graphite reflects most light -- that’s why pencil marks are so shiny -- and absorbs the rest, which is why it’s black.   But for a long time, these optical mark-sensing machines were very picky. And that’s where the different grades of pencil came in.   The graphite in pencils is held together by clay, so manufacturers can control how soft or hard it is, and also how dark it is. The softer the graphite, the darker the pencil’s mark. Pencils are labeled accordingly.   In the United States, those labels are numbers. International manufacturers use a different system, with HB corresponding to number 2.   Number 2 pencils make marks that are just dark enough to be sensed by the machine, but also light enough to be erased without leaving a readable mark.   These days, scoring machines use much more advanced sensors that can detect almost any sort of marks you make, as long as they’re in the bubbles.   So you could probably take your next exam with any kind of pencil you wanted. But your SATs probably aren’t the time to test that hypothesis. Maybe bring a couple of number 2’s just to be safe.   Thanks for asking, and thanks especially to all of our supporters on Patreon, where, if you support us at four dollars per month or more, you can submit your questions to be answered right here on SciShow Quick Questions. And if you want to keep getting smarter with us, don’t forget to go to youtube.com/scishow, and subscribe.