Why do we have a Supreme Court, and how does it decide cases?

Imagine a country full of rules — thousands of them, scribbled into laws over hundreds of years. But here's the puzzle: a rule on paper can mean two different things to two different people. So who gets the final word on what a law actually means? In the United States, that job lands on nine people in black robes. We call them the Supreme Court.
When the country was first built, the founders set up three teams to share power, so no single team could boss everyone around. One team makes the laws. One team carries them out. And the third team — the courts — decides what the laws mean when people argue. The Supreme Court sits at the very top of that third team. It's the last stop.
