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  • 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 instructors 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 machine.

  • 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 2s 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, and subscribe.

You probably know the drill: You're handed an exam, and you're told you need to fill out

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