An interjection is a word that expresses some kind of emotion. It can be used as filler. Interjections do not have a grammatical function in the sentence and are not related to the other parts of the sentence. If an interjection is omitted, the sentence still makes sense. It can stand alone.
When you are expressing a strong emotion, use an exclamation mark (!). A comma (,) can be used for a weaker emotion.
Interjections do the following:
Q. What should be B’s expression?
A: I got a perfect score on the math exam.
B: ____ (Well. Wow! or Um.)
Q. What should be C’s expression?
C: ____ ! (Nope, Hey, or Geez) My computer just broke.