Why do batteries blow up?

Why do batteries blow up? I was eating dinner the other night and kept hearing a very loud popping sound almost like a balloon popping but repeatedly. Finally I noticed that my 9V battery was blowing up ... whay does this happen? Where should I store the batteries so I know they won't blow up?

Try to keep them away from heat, and make sure that non-chargeable batteries are not being charged up by accident.

Heat will destroy a battery. Take the cover off the battery charger (break it if you have to), As rechargeable batteries get older they get hot! Don't set your battery charger on a cloth or rug. Keep it away from heat sources. Keep 'regular' batteries away from rechargeables do not mix them up in a device. For instance if you have a protable DVD player that takes 4 AA batteries don't put in 2 alkaline and 2 rechargeables. Or more then the batteries will blow up! On 9V batteries the + and - are next to each other. Do not let metal touch both leads at the same time. Don't put 9V or battery packs in your pocket full of change! I did that once... Just once!