What is the Constitution, and why is it called the supreme law of the land?

Imagine you and your friends are starting a club. Before anyone argues about snacks or rules, somebody writes down how the whole thing will work. That document — the rulebook everyone agrees to follow — is basically what a constitution is. The United States Constitution is exactly that, just for a whole country: the master rulebook for how the government works.
It was written in 1787, in a hot room in Philadelphia, by a group of representatives who argued, scribbled, crossed things out, and argued some more. They were trying to answer one big question: how do you build a government strong enough to work, but not so strong that it bullies its own people? Their answer became the Constitution.
