Is why a sentence?

"Why" is not a sentence. It is an interrogative word, also known as a question word. It is used to ask for a reason or explanation.

To be a sentence, "why" needs to be part of a complete thought with a subject and a verb. Here are some examples of sentences using "why":

* Why are you late?

* Why did the chicken cross the road?

* I don't know why she's crying.