Here's why:
* Phrase: A group of words that functions as a single unit but lacks a subject and verb.
* Prepositional Phrase: Begins with a preposition (like "because of") and ends with a noun or pronoun (the "object" of the preposition, like "her glittering smile").
Example: "He was happy because of her glittering smile."
The whole phrase "because of her glittering smile" acts as a single unit to modify "happy".