Dunno what made me think of this just now. When I worked for IT in a school district way back in the 90s, a librarian told me she kept a supply of mouse balls in her desk because kids would steal them out of the school computers. What I remember about those balls was they picked up dust and crud off surfaces. Pretty soon optical mice came along and they were history.
Today students pick keycaps off of keyboards and steal anything not tied down (so instead of just the ball, the entire mouse gets stolen). Once I heard a student ran off with an entire side panel of a computer