Here's why:

* Magma: Molten rock *beneath* the Earth's surface is called magma.

* Lava: When magma erupts onto the Earth's surface, it's called lava.

So, the process goes like this:

1. Magma forms deep within the Earth.

2. The magma rises through the Earth's crust.

3. If it reaches the surface, it erupts as lava.