Here's why:
* Participial phrase: A participial phrase is a group of words that includes a participle (a verb form ending in -ing or -ed) and its modifiers. It acts as an adjective, describing a noun or pronoun.
* Participle: In this sentence, the participle is "sleeping".
* Modifiers: The modifier is "in the corner".
The phrase "sleeping in the corner" describes the boy, acting as an adjective.