How do people become so rich?

I live in ‘small city’ (it’s not a big city, that’s for sure, but it’s also not small enough for us to know everybody), don’t call me stupid, but I never knew that my city would have ‘rich’ folks living in it. I found out that there were 2 kids in my school who are rich and live in mansions. How do people get so rich in the first place?

No criticism please. :P