Here's why:
* Interrogative: It introduces a question.
* Adverb: It modifies a verb, adjective, or another adverb. It tells us *how*, *when*, *where*, or *why* something happens.
For example:
* Why are you late? (modifies the verb "are")
* I don't understand why she left. (modifies the verb "left")
* Why are you so angry? (modifies the adjective "angry")
Let me know if you have any other grammar questions!