Answer:
But, my mouth was a sandy desert.
Explanation:
Not sure if there is another metaphor? is this the whole paragraph?
The subject is sponge because that is what is the main point of the sentence is.
Another term for introduction is an exposition.
exposition: the part of a play or work of fiction in which the background to the main conflict is introduced.
Put a photo up for more explanation.
A example of a fused sentence is: Heather read the novel her friend watched the movie