Here's why:
* It names a thing: "Water" refers to the substance itself, a clear liquid essential for life.
* It can be the subject of a sentence: "Water flows downhill."
* It can be the object of a verb: "I drink water."
However, "water" can sometimes function as a verb, meaning "to supply with water." For example:
* "The gardener watered the plants."
In this case, "watered" is the past tense of the verb "water."