Here's why:
* It describes an action: "Had" indicates possession or experience in the past. For example, "I had a great time" or "She had a dog."
* It can be conjugated: "Had" is the past participle of the verb "to have."
* It functions as a main verb in a sentence: The sentence "I had a good day" uses "had" as the main verb.
While "had" is often used in phrases like "I had a sandwich," "sandwich" is the noun, and "had" is still the verb.