Here's why:
* Common nouns are general names for people, places, things, or ideas. Examples include: dog, city, book, happiness.
* Demonstrative pronouns point out specific people, places, things, or ideas. They are used to show which one is being referred to. The most common demonstrative pronouns are: this, that, these, those.
Example:
* Common noun: "I saw a dog." (This is a general statement about dogs)
* Demonstrative pronoun: "I saw this dog." (This specifically identifies one particular dog)