Prepositions are words that require an object (usually a noun or pronoun) to complete them and show the relationship between that word and the object. A prepositional phrase is the combination of that preposition and its object. In the top tweet, Courtney opening lies inside (preposition) a steamy bath (object), therefore "inside a steamy bath" is a grammatically correct prepositional phrase. However, Courtney still has much to learn about prepositional phrases! The bottom tweet represents an awkward and technically grammatically incorrect use of a prepositional phrase. Remember, prepositions -- in this case, the word "within" -- require objects and while "a very sensuous Sunday morning" sounds delightful, it's not a quite an object per se. "Very sensuous Sunday morning sunlight" would have been better. For future reference, Court!