Like most batteries, 9V Batteries are sold at the supermarket and various other locations.
They are used in a huge variety of applications ranging from alarm clocks through calculators and even smoke detectors.
They are made up of 6 cells (usually).
These are smaller batteries (usually quadruple A batteries, or tiny carbon zinc batteries that are flat) and they are linked up to give the full power.
These are connected in series and as a result can be used individually if you want to.
Be aware though that they are not very powerful by themselves! To pull a 9V battery apart, simply get a knife and peel the metal foil off from around the side of the battery.
There shouldn't be anything dangerous inside of the battery, unless it is leaking acid.
Once you are inside of the battery you can pull them apart and do what you want with them.
If you have a rechargeable battery, people will often pull them apart when they stop functioning properly and replace the damaged cell.
Usually only one cell will go in these, which means there is potential to save a bit of money.
Generally though it's not worth playing with these batteries as they are so cheap anyway.
Rechargeable 9V Batteries are connected with nickel cadmium batteries, or lithium ion ones.
You do pay a bit more to get a rechargeable battery, but whether or not you do it should be justified by how often you use it.
For those who are still running smoke alarms with the 9V battery, seriously look into getting them hardwired in.
It will save you money in the long run, and be a much safer alternative!