Here's why:
* Prepositions show the relationship between a noun or pronoun and another word in the sentence. They typically describe location, time, direction, or manner.
* "Among" in this phrase acts as a prepositional phrase. It's a group of words that starts with a preposition and ends with a noun or pronoun. The entire phrase "among the weeping willows" functions as an adverbial phrase, modifying the verb implied in the sentence (for example, "walking among the weeping willows").
Therefore, "among" is the beginning of a prepositional phrase, not a preposition itself.