Here's why:
* It names a concept: Science refers to the systematic study of the natural world through observation, experimentation, and analysis.
* It can be the subject of a sentence: "Science is fascinating."
* It can be the object of a verb: "I love science."
However, it can also be used as an adjective in some contexts:
* Modifying a noun: "Science fiction movies"
* Describing a quality: "A science-based approach"
So, while "science" is primarily a noun, its grammatical function can sometimes be that of an adjective depending on the context.