The word "humming" can function as a verb (present participle form of the verb "to hum") or as a noun (gerund, representing the action of humming).
The part of speech for this particular word is a noun.
The part of speech for "answer" is a noun.
The part of speech for "explicit" is an adjective.
The part of speech for "twirl" is a verb.
The part of speech for "unfamiliar" is an adjective.
THEIR BUTT!!!
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The question you are asking is the same as a question like: Is building a house? or Is driving a car? You can built a house, you can drive a car and you can surely sing or hum a melody! Is singing or humming a melody, the act itself is not! But depending on what you are humming or what you are singing, the part that comes out of your mouth can be a melody...
part of speech
The part of speech for this particular word is a noun.
The part of speech for "answer" is a noun.
adverb
what part of speech is beneath
The part of speech for "explicit" is an adjective.
The part of speech for "twirl" is a verb.
The part of speech for "unfamiliar" is an adjective.
The part of speech of "came" is a verb.