Here's why:

* Iron and nickel primarily make up the Earth's core, the innermost layer.

* The asthenosphere is a layer within the upper mantle, which sits above the core.

* The asthenosphere is composed of solid rock, but it behaves like a viscous fluid due to its high temperature and pressure. It's made up of minerals like olivine and pyroxene.

Let me know if you have any other questions about Earth's layers!