It's a common noun, meaning it refers to a general thing, and it's a concrete noun, meaning it refers to something tangible.
For example:
* The sky is blue today.
* We watched the sky for shooting stars.
* The birds flew high in the sky.
It's a common noun, meaning it refers to a general thing, and it's a concrete noun, meaning it refers to something tangible.
For example:
* The sky is blue today.
* We watched the sky for shooting stars.
* The birds flew high in the sky.