a goat eating falafels
Nintendo Wii DVD Drive
iPhone 4S Dock Connector
I had this problem too! usually, you can find the little black circle stuck on one of the adhesive pads near the volume buttons and stick it back on using tweezers and a magnifying glass.
If you can't find it, you can cut a sticker so that it's about the same size as the little black dot, then reassemble as usual: )
the battery powers the internal clock
so that you don't have to reset the time every time you plug it in :)