Here's why:

* Location: The asthenosphere is a part of the upper mantle, located beneath the lithosphere (the rigid outer layer of the Earth).

* Properties: It is characterized by its partially molten state, meaning it has some solid rock but also has a significant amount of molten material. This partially molten state gives it a plastic-like consistency, allowing it to flow very slowly over geological time.

* Significance: The asthenosphere's ability to flow is crucial for plate tectonics. The rigid lithosphere plates "float" on the asthenosphere and move due to the convection currents within the asthenosphere.

While the asthenosphere is considered the softest layer, it's important to remember that it's still incredibly dense and under immense pressure. It's not soft like a pillow, but rather more like a very viscous fluid.