How, exactly, does "money-trading" make more money?

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Sorry if I sound stupid for asking this, but I feel that I don't truly understand it that well... You know the way we currently make and have been making money all over the world? Money trading i.e. you use a $20 bill to buy something for $5 at the food stand and that $5 pays for the food stand guys bills or something the money ends up all around the world eventually and somehow makes more money in return? How exactly does that make more money when the govt. could easily just make more bills with a money printing machine? Could anyone please break it down? Would appreciate it. Thanks.