What Causes a Volcano to Erupt?

Volcanoes erupt because pressure builds up inside the Earth and then suddenly lets out a big 'whoosh!' Imagine you're filling a bottle with soda, if you shake it up and open it too fast, it all spills out at once. That's kind of like what happens in a volcano: hot rock and gas build up until they can't stay in anymore, and poof! everything comes rushing out.