A hard rubbery ball which rests within a cavity in the underside of the mouse. It rolls around when the mose moves and transfers the movement to small wheels which convert the movement into signals which are passed to the computer.

In most computer mice, these make contact with the mouse pad and move the x and y rollers inside the mouse, generating cursor movement. In well made mice, they are heavy (about thirty grams) and coated with slightly tacky rubber. In cheaper mice, they are lighter weight and have a matte black coating. Optical mice have an LED instead of any gauche moving parts.

A local used computer store went out of business a few years ago, and sold off all of their merchandise for something like seventy percent off. Among the stuff offered for sale was a white box marked mouse balls on all six sides in red marker. I bought it immediately, and spent my entire walk home giggling. Nowadays the box sits in my room, and every once in a while a guest gives in to temptation and opens it. They are invariably less impressed with the contents than they thought they would be.

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