I was swishing around my favorite mouse (the Logitech MX300, no longer made) and was thinking Wow, the action on this mouse is much smoother than I remember so I flipped it over, and saw that a FELT MADE OF CAT HAIR had formed where the old adhesive had held the original plastic runners. I am grossed-out yet oddly pleased at this organic development; as I said, the mouse is working pretty well. Just don’t look underneath it.