Here's why:
* Adjectives describe nouns: "Funny" describes a noun, like a "funny story" or a "funny person."
* Parts of speech categorize words based on their function: Nouns, verbs, adjectives, adverbs, pronouns, prepositions, conjunctions, interjections are all parts of speech.
Let me know if you'd like more examples of how "funny" works as an adjective!