Here's why:
* Prepositions show the relationship between a noun or pronoun and another word in the sentence. They often indicate location, time, or direction. Examples include "on," "in," "at," "to," "from," "with," "by," etc.
* Adverbs modify verbs, adjectives, or other adverbs. They tell us how, when, where, or to what extent something happens.
In the phrase "I talked about the weather," "about" tells us more about the verb "talked." It explains what was talked about.
Therefore, "about" functions as an adverb in this example.