Here's why:
* Adjective: It describes or modifies a noun.
* Example: "The instructions were simple." (Simple describes the instructions)
* Noun: It names a person, place, thing, or idea.
* Example: "The simplicity of the design was impressive." (Simplicity is the noun)
While "simple" itself isn't a noun, it can be used as a noun in certain contexts, like:
* "The simple life is what I crave." (Here, "simple" acts as a noun referring to a lifestyle)
But in most cases, "simple" will be functioning as an adjective.