What part of speech is fastest-changing?

This is a bit of a tricky question, as "fastest-changing" isn't a traditional part of speech.

Here's how we can break it down:

* "Fastest" is an adjective. It modifies the noun "changing".

* "Changing" is a participle acting as an adjective. It modifies the noun "part of speech".

Therefore, "fastest-changing" is a compound adjective. It describes the noun "part of speech" and indicates that it is changing quickly.

It's not about a specific part of speech being more prone to change, but rather about the rate of change within a specific area of language. For example, you might say:

* "English is the fastest-changing language in the world."

Here, "fastest-changing" describes the noun "language", indicating that English is changing at a rapid pace compared to other languages.

So, "fastest-changing" isn't a part of speech itself, but rather a description of something (like a language or a particular aspect of language) that is changing quickly.