From the University of Ottawa: "Traditional grammar classifies words based on eight parts of speech: the verb, the noun, the pronoun, the adjective, the adverb, the preposition, the conjunction, and the interjection. Each part of speech explains not what the word is, but how the word is used. In fact, the same word can be a noun in one sentence and a verb or adjective in the next."
Often, a word first appears as a noun, and then becomes a verb. "Party" is a notable recent example. "Party on, Garth.
As a grammar geek, I am particularly fascinated by the transition of the word "street."
It started as a noun, and has worked as an adjective ("street clothes") and adverb ("he talks street"), and now functions as at least two kinds of verb:
When there's not enough evidence to hold a suspect, the precinct lieutenant or captain may tell the detectives, "We'll have to street him," meaning release him so he can go out on the street.
In retailing, the "street price" is a typical selling price for an item "on the street" -- usually lower than the suggested retail price. A sales manager might say, "The list price for our new KZR-202L is 799.95, but it will probably street for $699."
In video games, music and movies, the "street date" is the date when a new release is allowed to be sold "on the street." The sales manager might say, "The street date for the 3D Blu-Ray is December 10."
(photos from Google Images [photographer unknown], NBC, Disney)