Another argument for storing batteries in plastic containers...

It's almost impossible to accidentally short an AA-style battery. A 9 volt, on the other hand, needs nothing but brief contact with a screwdriver, nail, etc. or the side of the drawer in a metal tool box.

Many spare drill batteries have contacts so oriented that accidental shorting is a possibility. A little plastic goes a long way to preventing fires.

Oh, and one more warning...never carry 9 volt batteries in your change pocket. :-)