One other note is to be careful when throwing out 9V batteries, some house fires have been traced to these culprits.
Safety experts recommend placing electrical tape on the terminals to prevent a short. One expert even suggested adding the electrical tape to the battery case before installing it so you'll remember to tape over the terminal posts when removing it.
Another potential area is the family junk drawer where unused batteries, keys, tools... are often tossed. 9V batteries have been known to combust when a key, pen clip or paper clip shorts the terminals or even two batteries touching terminal to terminal.