What is the prepositional phrase in sentence The cold brook bubbled merrily as it tumbled over smooth gray stones?

Here's why:

* Prepositional phrases start with a preposition (like "over," "in," "on," "at," etc.) and are followed by a noun or pronoun (the object of the preposition).

* "Over" is the preposition.

* "smooth gray stones" is the object of the preposition.

The prepositional phrase tells us where the brook was tumbling.