Why Do Political Parties Change Their Ideas Over Time?

Imagine a group of friends who start a club to build the best treehouse ever. They all agree on what they want it to look like. But as time passes, some people want a slide instead of stairs, and others want more windows than walls. Eventually, the treehouse doesn’t look anything like the one they originally imagined. Political parties are kind of like that group of friends, except their treehouse is the government, and the rules change over time.