Why Do Volcanoes Sleep for Centuries—and Then Explode?

Volcanoes are like sleeping giants. They rest for a long time, but when they get tired of being quiet, they erupt! Inside the mountain is a big chamber filled with magma, which is hot liquid rock. When more magma builds up and pushes hard on the rocks around it, the volcano wakes up and explodes. Sometimes this happens after a big earthquake, like when a giant gives a sleepy giant a shake.