If you can light steel wool with a match, why can't you light a nail? Or a cast-iron pot? It's a question of surface area versus volume. The burning process, which is just rapid oxidation of the metal, has to bring nearby iron to its ignition temperature fast enough to sustain a chain reaction. Thick pieces of iron conduct heat away far too fast for the surface to ever reach the ignition point. But in very thin strands, there's nowhere for the heat to go, and a burning patch can race along the length of wire, converting a whole steel-wool pad into iron oxide-rust-in less than a minute.