Is why a sentence?

"Why" is not a sentence. It's an interrogative word (a word used to ask a question).

Here's why:

* Sentences need a subject and a verb. "Why" doesn't have either of those.

* Sentences express a complete thought. "Why" just asks for information; it doesn't make a statement.

Examples:

* Incorrect: Why?

* Correct: Why are you late? (This has a subject "you" and a verb "are")