neowin[.]net/news/a-quick-look-back-at-the-reveal-of-the-microsoft-intellimouse-explorer-25-years-ago-today/
I've still got mine, and it still works [better than my body after 25 years]. :)
Just in case you might be unfamiliar with why the IntelliMouse Explorer is so important in the development of PC accessories, the IntelliMouse Explorer was the first one made for the home PC market with an optical sensor. While it was not the first mouse product with that kind of technology, the IntelliMouse Explorer was certainly the first that had an optical sensor instead of the normal embedded mouse ball that was made for a mass-market audience.
For those youngsters who never experienced the old mice [and trackballs], you missed all sorts of fun... when you moved the mouse [or trackball] the ball turned 3 rubber wheels connected to sensors. The mouse ball was rubber coated, so it picked up all sorts of gunk that it deposited on those 3 rubber wheels. With a trackball it was mostly sweat & gunk from your hands. So Very regular maintenance was popping out the ball & cleaning everything out using Q-Tips etc.