Why Do Volcanoes Explode?

Imagine a volcano like a pressure cooker. When hot rock (called magma) moves up from deep inside the Earth, it fills the space below the volcano. If the magma is runny and has lots of gas bubbles, it can pop out suddenly, that's an explosion! But if the magma is thick or there’s not much gas, it just oozes out slowly like honey.

Sometimes volcanoes explode with fire and smoke, and other times they just bubble quietly. That depends on what kind of magma is inside them.