Here's why:
* It describes a noun: We use "happy" to tell us something about a person, place, or thing. For example, "The happy child laughed."
* It doesn't act as a verb: We don't use "happy" to show an action.
* It doesn't connect words or phrases: It doesn't have the function of a conjunction or preposition.
Let me know if you'd like to explore other parts of speech!