See Reaction That Occurs When 9-Volt Battery Makes Contact With Steel Wool

You may want to avoid doing this science experiment at home.

A combustive reaction takes place when a 9-volt battery is brushed up against a common steel wool pad.

The electrical current from the battery heats up the wire which then reacts with the oxygen surrounding the steel wool. This generates an initial spark that creates a chain reaction causing the entire pad to burn.

Here's a more thorough explanation from the BBC:

How can a battery set it on fire?

Metals conduct electricity very well, which is what happens when you touch the positive and negative terminals of a battery against the steel wool. Lots of electrical current can flow along the short lengths of steel wool which connect the terminals – you’re making what’s called a short-circuit.

Electrical current is actually just a flow of charged particles. When a lot of these move around in the wire, they basically collide with the atoms in the metal, which slows them down (it’s called electrical resistance).

Those collisions also produce heat. With a large current flowing down a small enough wire, there’s enough heat produced to set fire to things. In fact, the hotter the wire, the greater the resistance, so the wire gets hotter and hotter until it melts apart.

A similar thing happens when you use electrical items: they get hot. However, unless there’s a short-circuit, or a defect so that all the current is flowing down one thin wire, most electrical items shouldn’t get as hot as in our experiment. That’s because they are designed to have much thicker wires for a given amount of electrical current.

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