What's this stuff on my computer mouse?

What's the stuff that builds up on the underside of a computer mouse?

• You know, that near-black cruft you need to scrape off every now and then. Does it come from the mouse? From the environment? From me?

• Answer:

  It's informally known as mouse shit. It's mostly comprised of dead skin particles and little flakes of hair. These fall off you as you use your desk. The motion of your wrist and the base of your palm upon the desk then leave sweat and grease behind that coagulates these particles into the black greasy stuff you're seeing. So, if you want to avoid it, force yourself to keep your wrists off the desk surface or clean your hands religiously. Alternatively, if you use a mouse pad, you might find the printing ridges in its surface collect the stuff better than the mouse itself does.