Sandpaper is a special thick paper with a rough, abrasive surface. Painters will often sand a wall with sandpaper before painting it.
Woodworkers use sandpaper to smooth wooden surfaces, and sandpaper is also useful for removing loose paint, grit, or dirt when you're painting or refinishing something. Sandpaper is thought to have been invented in thirteenth century China, from a crushed substance, often shells or sand, that was glued to parchment with natural gum. In the 1800s, a similar product called "glass paper" was made from crushed glass, which, despite its name, is sometimes still used in today's sandpaper.