Fingernail.

I will always use ball mice from now on. I had an optical for a while, but it seemed to go berzerk at random times and the cursor would jump to one corner of the screen, far off the edge of the screen, so I had to pull it back onscreen. Very annoying. So ball mice for me.

(Off topic, this seems like the appropriate place to crack a joke about not having any "balls"... but I am not that much of a nerd to actually know a joke about that.)