Believe it or not, 9-volt batteries can be a safety hazard under certain circumstances. The batteries that power toys, household devices, and (ironically) smoke detectors can be a fire hazard.
The problem is the positive and negative metal posts. Any metal object (key, paper clip, coin, steel wool) touching both metal poles at the same time can create a short circuit. Infrequently, this can generate enough heat to start a fire. Even discarded weak batteries thrown in the trash with metal objects have started fires.
The solution is simple in theory: Keep metal objects from bridging the metal poles on 9-volt batteries. In practice, we toss 9-volt batteries in drawers along with pens, paper clips, paper and everything imaginable.
The National Fire Protection Association (NFPA) recommends storing 9-volt batteries in original packaging until used. If loose, cover the posts with electrical, masking, or duct tape and keep them from contacting metal objects. Put them somewhere where they wont be bounced around, and store them standing up in containers separate from other batteries.
Before disposing of 9-volt batteries, cover the positive/negative posts with electrical, masking, or duct tape. The NFPA recommends not throwing into regular trash, and disposing at a household hazardous waste collection site.
Source: http://www.nfpa.org/safety-information/safety-tip-sheets
