The genie chuckled slightly trying to avoid looking either of you in the eye. "Ha, ha, ha! I play honestly so the problem must be with your wish."
"How?" Lydia asks. "You said temporary. He wished for temporary. The changes should have been temporary."
"They are," the genie said appearing to float into a chair. "You changed then you went back to normal. Temporary."
"But then she started changing again. That would suggest permanent," you reason. Suddenly you felt like you were a pointy eared character reasoning with a super advanced alien. Magic should not have to be logical.
"If the magic is still with Lydia," the genie grinned, "then the other part of the wish must still be active."
You look at Lydia questioning. She looks back at you also searching for the answer. Temporarily turn into a mermaid; done. Make love to her; done. What other part of the wish was there?
"Genie," you say, "we have made love. What other part is there?"
"You wished to fulfill your fantasy," and his grin grew a little. "Have you done that?"
"Yes," you say with conviction.
"Really?" The genie leaned forward.
"Yes," you say with a little less conviction.
"Then you are completely finished with making love to mermaid-girlfriends?"
The sudden realization hit you dumb. Your mouth flapped a little while you imagine life over the next twenty years. You and Lydia on remote islands having sex on the beach. Long week-ends spent by the pool watching the busty, slick mermaid lounging and swimming. There were so many places and so many colours you still wanted to see. You wanted to take pictures of Lydia and put them on the internet. You imagined Lydia needing you to help her keep the secret from people she knew. Television-like plots to sneak a partial mermaid out of awkward situations floated through your mind. Every time it would end with the two of you exhausted in bed declaring how much you love each other. It was all part of your fantasy.
"No," you say finally. "My fantasy is not fulfilled yet. It might never be."
Lydia seemed shocked. The genie nodded slowly. "You asked for your girlfriend to be a mermaid. As long as Lydia is your girlfriend or more she will continue trying to satiate your fantasy."
"Just like at lunch," you muse. There was no reason why she should have made the mistake of fingering the water on the table except to create a situation for you to solve. Lydia would probably continue to initiate public transformations for you to save her from. It was all part of your fantasy.
"And you said 'girlfriend' not 'Lydia.' It will follow you not her if you two split. The next one might not like it as much you know." The genie laughed now. It was deep and long and not without a little malice.
Lydia pipped up then, "Please, sir, what causes me to change?"
"The combination of swimwear and contact with water. You can control the rate of change and the suit you wear but once it is started you will be unable to stop it until it is completed with sex. You start it, he finishes it. Be wise."
Lydia looked down at her altered form. The different coloured skin and inflated breasts. The stiff legs and the dyed hair. If she was not careful she would find herself partially changed and waiting for her boyfriend to free her. She never felt so dependent before now. She liked her potential changes but she hated the fact that it was bound to someone else. Lydia was jealous of her change and wanted full control. If her boyfriend left her she would not only loose a friend but also this unique body.
"Now that I have answered your questions; answer one of mine. Whomever summons me can have a wish to change either themselves or someone else, either permanently or temporarily. What do you wish?"