Case-hardening

Case-hardening is like wearing armor for metal objects. Just like knights wear armor to protect themselves from swords, metal objects like gears, bolts, and tools can be made tougher and more resistant to wear and tear by adding a hard outer layer.

To do this, we first heat up the metal object to a very high temperature. Then we introduce carbon (a material found in things like coal and diamonds) to the surface of the metal. The carbon reacts with the metal to create a new, harder layer on top called "case," like a shell.

The case is hard and tough, but the metal underneath is still soft. This is good because if the metal were hard all the way through, it would become brittle and break easily. The hard outer layer protects the softer metal inside, making the object last longer and do its job better.

So, imagine that you have a toy car made of metal and you always play with it by driving it over rocks and rough surfaces. If the toy car were case-hardened, its surface would be hard like a rock, protecting it from scratches and dents. Therefore, your favorite toy car would be more durable to play with.