Here's why:
* Participial Phrases: A participial phrase is a group of words that acts like an adjective and includes a participle (a verb form ending in -ing or -ed).
* Participle: In this sentence, the participle is "kept".
* Function: The phrase "kept on a sunny windowsill" describes the violet, acting as an adjective.
Let me know if you have any other sentences you'd like to analyze!