Interjections are words or phrases used to convey surprise, as well as emotions such as joy, shock, excitement, fear… the list truly goes on and on.
An interjection can be followed by an exclamation point, question mark, period, or comma. It all depends on whether it stands on its own or if it's part of a sentence—and how dramatic you want to sound.
Examples of interjections include:
Yeah, they look kind of sad without exclamation points. But, hey, maybe they are sad. You don't know their lives.
"Oh, I thought everybody was going to the party dressed as their favorite Hunger Games character. Now I just feel like a dork carrying around this bow and arrow."
In this example, the interjection oh is used to express surprise. Sure, the speaker feels silly now, but if the party is attacked by muttations, those arrows will come in very handy.
"You can eat seven saltines in one minute? No way!"
Consuming seven saltines in sixty seconds is virtually impossible unless you're Peyton Manning. On the other hand, it's totally possible to identify no way as an interjection in this example. Here it's being used to express the speaker's disbelief.
"Holy guacamole! Susan's new haircut makes her look like Justin Bieber circa 2009."
Here, the phrase holy guacamole is an interjection used to express surprise, or perhaps shock, at the fact that anyone would pay a stylist to make them look like one of Canada's most infamous musical exports.