Here's why:
* Subject: The subject of a sentence is the person, place, or thing that performs the action.
* Predicate: The predicate tells us what the subject is doing or what is happening to the subject.
In this phrase, we don't know who is blowing the whistle. To complete the sentence, we need a subject, like:
* The referee blew her whistle. (Here, "the referee" is the subject, and "blew her whistle" is the predicate.)
* The train blew its whistle. (Here, "the train" is the subject, and "blew its whistle" is the predicate.)
Let me know if you have any other sentences you'd like to analyze!