With what begins a prepositional phrase? (please give an example of prepositional phrase)

A preposition is a word that shows location, such as
in, on, under, at, and besides.  These little words occur
in many different sentences and can be very confusing to English language
learners.

A prepositional phrase is a longer group of words
that uses a preposition along with a noun or pronoun to give a better description.  The
phrase functions as a single entity and does not contain a subject and a
verb.

Prepositional phrases can be located in the
beginning, middle or end of a sentence.

For
example:

The boy sat
besides the swimming pool dreaming of the girl in his 6th
period class.  To identify the preposition, ask yourself
where the boy sat.  Obviously, the answer is besides the swimming
pool.  Notice that besides the swimming pool is
not a complete thought; therefore, it is a phrase.  The word
besides shows location and makes the phrase a prepositional
phrase.

 Sometimes, a prepositional phrase may
occur at the beginning of a sentence.

For
example:

In the middle of his presentation,
Steven realized that he had forgotten to turn off his
stove.

Alternatively, the sentence can be rearranged and
the prepositional phrase can be placed at the end.

For
example:

Brian picked up his backpack and stormed
out of the door.