Book 3, Question 132

Why Does Sandpaper Scratch?

Sandpaper is a crowd of tiny hard teeth, glued onto paper or cloth. Each tooth plows a microscopic groove and carries away a speck of wood or paint!

From the book

Page from A Million Whys Book 3

Sandpaper is a single layer of hard, gritty grains glued to a flexible backing.
The name is a bit of a fib. Most sandpaper has neither sand nor paper in it.
As far back as the 1200s, people in China glued crushed shells and seeds to parchment to make an early sandpaper.
By 1769, abrasive paper was being sold on the streets of Paris.
In 1835, a United States patent was issued for a machine that made sandpaper.
Grit numbers tell grain size. 60 grit looks like sugar grains, while 2,000 grit looks like flour.

What do the numbers on sandpaper mean?

They show grain size. Low numbers like 60 are coarse, and high numbers like 2,000 are very fine.