Usually, the words that constitute a phrasal verb are a verb and a preposition, but that is not always the case. Sometimes the first word in a phrasal verb is not a verb at all, but when paired with the preposition, the whole phrase becomes a verb. For example, the phrasal verb “clam up” is made of a noun (clam) and a preposition (up). When you combine them, however, they become one verb meaning “to become quiet or refuse to speak.”