Pencils are writing instruments made primarily of a core of graphite encased in wood or plastic. They are used for drawing, writing, and marking. The hardness of the graphite can vary, which affects the darkness of the marks made on paper. Pencils are often graded with a scale ranging from soft (B) to hard (H), with HB being a common middle grade. Pencils come in various shapes and sizes, including standard wooden pencils, mechanical pencils, and colored pencils. They are popular in schools and offices due to their ease of use and the ability to erase mistakes. The eraser at the end of many pencils allows for corrections without leaving a permanent mark.