Famously described as a "noble lie" by Socrates (414c), the myth of metals demonstrates how lying, if it's for the good of a city, can actually be a good thing for ruler to do. (So says Plato, anyway.)
The story goes that once upon time, Mother Nature invented all human beings and mixed different metals—gold, silver, bronze or iron—into their characters. Rulers have gold in their characters, workers have iron, and so on for all different classes and professions.
Obviously not true, right? Not important. Plato imagines that a nice, simple little story like this will make it easier to rule since 1) everyone will think of their social class as an extension of their inner character (so no complaining), and 2) everyone will know they have a common mother, so they will treat each other peacefully.
You probably know that the Greek world was filled with myths, so it's especially interesting to see here how Plato is acknowledging, and exploiting, the fact that myths are just stories people make up. Did we mention that Socrates was sentenced to death for impiety to the gods?
Yeah. This kind of stuff doesn't make everyone super happy.