Blacksmith
To heat-treat steel, a blacksmith heats the steel until it no longer pulls a magnet, then makes the steel become cold very quickly. A blacksmith does this by putting the hot steel into a bucket of water and moving it around until it is cold. This is called "quenching". When this is finished, the steel will be as hard as it can be. This hardness can make it brittle, so if someone hits or drops it, it can break like glass.