with just some pliers and a good quality 9v battery you can open a 9v battery without damaging the internal cells so they can be used as a replacement AAA battery
oh and by the way THIS IS NOT DANGEROUS you are not opening up a cell you are opening up a battery (batteries have cells joined together)
Step 1: Gather Your Tools and Materials
go and grab your pliers and a good quality battery. I use a energizer battery but a Duracell might do it as well.
Step 2: Somewhere to Start
get your pliers and pinch the middle of one of the long sides then bend th metal back, don't worry about the dints in the metal it makes just keep going. you may have to do this a couple of times to get the metal lip up but when you do things suddenly get easier.
Step 3: Keep Going
keep going along the long side lifting the lip up until you get to the corners.
Step 4: Corners
corners are a bit trickier than the rest of the battery. use the corner of the battery that is closer to the fold in the steel. lift the lip of the other side then do the corner, usually the corner sort of scrunches up a bit but this is normal.
Step 5: Nearly There...
keep going on that shorter piece you started on and then do the opposite corner.
Step 6: Lift It
do the 3rd corner on one side and it should start to lift. pull it out after this happens.
Step 7: Those Little Extras
you will find at this point that there is a little piece of paper in there. pull that out to reveal the batteries.
Step 8: Shake, Shake, Shake!
give the battery cells inside a good shake and all of the cells should fall out along with the bottom. if you open up the bottom layered sort of thing you will find this silicon layer, i have no idea what it might be used for but keep it anyway.