Here's why:
* Common nouns are words that name general things, places, or people (e.g., dog, city, teacher).
* Adjectives are words that describe nouns (e.g., great, big, blue).
In a sentence, "great" would modify a noun:
* The Great Wall of China is amazing. ("Great" describes the noun "Wall").
Let me know if you have any other words you'd like to classify!