Why do revolutions happen when people want to change their government?

Here's a curious thing about countries: most of the time, people grumble about their leaders and then go back to their soup. But every so often, the grumbling turns into something enormous — a revolution, where a whole society decides its government has to go. So what flips the switch? Why does ordinary frustration sometimes become history? Let's pull the lever and look inside.
Start with a simple idea: a government is really a deal. The people agree to follow the rules, and in return the government is supposed to give them safety, fairness, and a say. Most of the time the deal feels good enough to keep. A revolution begins when huge numbers of people start to feel the deal is broken — and that no one in charge intends to fix it.
