Here's why:
* Prepositional phrase: It consists of a preposition ("off") followed by a noun phrase ("the ground").
* Adverb: It modifies a verb, adjective, or another adverb, telling us *how, when, where, or to what extent* something happens.
Example:
* The plane lifted off the ground. ("Off the ground" tells us *where* the plane lifted.)
Let me know if you have any more questions!