Here's how to think about it:
* Compound noun: A noun formed by combining two or more words.
* Separate words: The individual words that are combined to form the compound noun.
Examples:
* "Fireplace" is a compound noun made up of the separate words "fire" and "place".
* "Toothbrush" is a compound noun made up of the separate words "tooth" and "brush".
* "High school" is a compound noun made up of the separate words "high" and "school".
Note: Sometimes the separate words in a compound noun are written as one word (like "fireplace"), while other times they are written as two words (like "high school"). The way they are written doesn't change the fact that they are separate words that make up the compound noun.