Old-school non-optical mouse
The new school just don't have a mechanical component of a ball and two rollers which spin two separate "gears" which blocks light and then math to figure out the horizontal change + vertical change.
I found that pretty neat when I learned. (I'm old enough to remember the pains of using a gunked up ball mouse.)
See this: https://commons.wikimedia.org/wiki/File:Sun_optical_mouse.jp...
That's not a regular mouse pad, it's a special red/blue/silver grid detected by a laser in the mouse.
Edit: At least on a Sun workstation