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," etc.
* "So" is an adverb. It modifies a verb, adjective, or another adverb, expressing intensity, degree, or consequence.
Examples:
* Preposition: "The book is on the table." ("On" shows the location of the book.)
* Adverb: "I am so tired." ("So" modifies the adjective "tired" to indicate intensity.)
Let me know if you have any other grammar questions!