How is whoa a sentence?

"Whoa" isn't a sentence on its own. It's an interjection, which is a word or phrase that expresses strong emotion.

Here's why it's not a sentence:

* No subject: Sentences need a subject (who or what is doing the action). "Whoa" doesn't have one.

* No verb: Sentences need a verb (the action). "Whoa" doesn't have one.

* No complete thought: Sentences express a complete thought. While "Whoa" can convey surprise or excitement, it doesn't form a complete thought.

You can use "whoa" in a sentence like this:

"Whoa! That was a close call!"