Here's why:

* Magma: This is the name for molten rock found beneath the Earth's surface.

* Lava: This is the name for magma that has erupted onto the Earth's surface.

While the mantle itself is mostly solid rock, it behaves like a very viscous fluid over long periods due to the intense heat and pressure. This allows for the movement of magma within the mantle, which drives plate tectonics and volcanic activity.