An interjection is a word or phrase that expresses strong emotion.
Examples:
* Wow! That's amazing!
* Ouch! That hurts!
* Hey! Watch out!
However, exclamation marks can also be used to emphasize other parts of speech, like:
* Nouns: The Eiffel Tower!
* Verbs: Stop that!
* Adjectives: It's beautiful!
The key thing is that the exclamation mark is used to emphasize or express strong emotion, regardless of the specific part of speech.