Also from reader JV, a nifty little tutorial on extracting AAA batteries from a 9V.
With a needlenose pliers, gently peel away the metal cover of the 9 volt battery. It should separate along the seam without much effort. Be careful though, because the metal edges can be sharp!
I remember thinking at one point that a 9 volt battery was just a small box full of acid or something, but as you can clearly see, this one is is just constructed of six smaller 1.5 volt cells wired in series.
I'm sure this will come in handy at some point in my life.