Typically when I clean out an old, dirty keyboard, I find things like bits of decomposing food, lots of lint/hair/dust, and one or more rusty paperclips.
I also discover many switches hiding under my keys. Sometimes they are blue, other times they're brown or black, and SOMETIMES there's just a big ol' spring there. Not quite sure what their function is, but I'll get back to you when I find out more about it... #sarcasm