Here's why:
* Prepositional phrases consist of a preposition followed by its object (e.g., "in the house," "with a smile," "after dinner").
* Subordinating conjunctions connect a dependent clause (which cannot stand alone as a sentence) to an independent clause (which can stand alone).
Example:
* While I was sleeping, the phone rang.
In this sentence:
* "While I was sleeping" is a dependent clause.
* "The phone rang" is an independent clause.
* "While" connects these clauses and indicates a relationship of time.
Therefore, "while" functions as a subordinating conjunction, not a prepositional phrase.