Here's why:
* Prepositional Phrases: A prepositional phrase starts with a preposition (like "after," "before," "during," "with," etc.) and ends with a noun or pronoun (the object of the preposition).
* In this case:
* Preposition: "after"
* Object of the Preposition: "a heated discussion"
Therefore, "after a heated discussion" acts as a single unit, modifying the verb or noun it follows.