Interjections are words or phrases that express strong emotion or surprise. They are typically used on their own, rather than part of a sentence.
Here's why "eureka" is an interjection:
* It expresses strong emotion: It's used to convey excitement or triumph when a discovery is made.
* It stands alone: You wouldn't typically find "eureka" being used within a sentence as a noun, verb, adjective, etc.
Let me know if you'd like to know more about parts of speech!