Before your children went back to school, did you spend time picking out folders, notebooks, art boxes, lunch boxes, and maybe a few No. 2 pencils? Have you or your kids ever wondered why No. 2 pencils are always on a teacher’s classroom shopping list, or why they are the only type of pencils allowed on a standardized test? We've compiled the most comprehensive answer for you, courtesy of Mental_Floss and Dictionary.com. 

Let’s begin by setting straight a common misconception: Pencils do not, in fact, contain lead. The points of pencils are actually made from a mixture of graphite and clay. The ratio of the two determines the hardness of the pencil’s “lead,” and the number identifies that hardness. The higher the number on a pencil, the harder the pencil.

According to Dictionary.com, early machines that scanned and recorded test scores couldn’t properly detect marks made by hard pencils. While No. 1 -- the softest -- pencils were prone to smudging, the No. 2 pencil was the perfect balance of softness and hardness.

Though new machines are getting better at correctly scanning other types of writing utensils, we suggest sticking with a good ole’ No. 2 pencil until you’re instructed otherwise. Better safe than sorry!