fun - part of speech

Compared to other languages, English is in practice pretty indifferent with regards to parts of speech. The lines are often blurry. I'm curious about the following phrase:

It's fun.

Usually, I'd think that "fun" is a noun. For instance I can have lots of it. However, there are phrasings where it is used much like a qualifier.

It was a fun ride.

Just looking at the structure, it could be either an adjective or a noun in the phrase in question.

It's fun/milk. It's fun/nice.

My first impulse was "noun" but seeing how there is no article and how most phrasings that express roughly the same would use an adjective, I am not so sure anymore. Here are my questions.

• What part of speech is "fun" in the phrase?
• Do native speakers clearly perceive it as noun or adjective respectively or would both readings make sense on an intuition level?

Indifferent is not the correct word to describe the way native English speakers regard parts of speech; it's more like flexible:

In the unambiguous expression:

the children enjoyed the fun and games

Fun is a noun.

In the unambiguous expression:

the children enjoyed the fun games

Fun is an adjective.

In the ambiguous expression:

it's fun

The larger context will determine the part of speech:

Fun is renaming it as a predicate noun.

or

Fun is describing it as a predicate adjective.

As long as people can perceive, use and interpret parts of speech properly, it is not so important to label them. We teach our children to label parts of speech so that they can learn to speak, read and write properly. As instructors, the labels remain supremely relevant to us, but once our students can speak, read and write, the labels quickly become less relevant to them. Those who never discuss parts of speech, tend to forget the significance of the labels, because they don't need the labels to communicate.

The adverb "very" can modify adjectives, so if "fun" in "It's fun" were an adjective, you would be able to say *"It's very fun." But you can't (at least, not in my opinion). That would have to be expressed as "It's a lot of fun", which has "fun" as object of a preposition and so a noun. So, I think the "fun" you asked about is a noun and not an adjective.

Elsewhere, "fun" is an adjective and can be modified by "very": "The carousel is a very fun ride", where "ride" is a noun modified by the adjective "fun" and "fun" is modified by "very".

Nouns can be used as compound elements for compound nouns as in fruit tree, oil well etc. If it comes to describing compound nouns consisting of two nouns I would not use the term adjective for the first noun. This is a confusion of terms. There should be special terms for word classes, word groups, sentence structures and word composition. If one term is used in several categories this is unclear terminology.