Why do exchange rates between countries change?

You wake up one morning and check the news: the dollar is "up" against the euro, "down" against the yen, and nobody's quite sure what's happening with the pound. What does that even mean? And why can't currencies just pick a number and stick with it?
Think of currencies like tickets at an arcade. You walk in with dollars, trade them for arcade tickets, play games, win prizes. Simple. But here's the thing: the arcade sets the exchange rate. "Today, one dollar gets you four tickets." Tomorrow? Maybe five tickets. Maybe three. The arcade decides based on how badly it needs dollars versus how many tickets it printed.
