You decide to wish to be a merfolk, because those were the only known creatures you liked. "Genie, I wish to become a merfolk creature," You say. "The people that are human at the top with head chest, and arms, while the rest would be fishy." "Very well," the genie says. "I know what you're talking about."
The genie snaps her fingers, and teleports you, and herself into an ocean where you see dozens of see creatures. The first thing that happens to you is that your clothes vanish, and gills open around your neck on both ends. "Merpeople need gills to breathe in this new territory," the genie explains. "Now for the next question. Do you wish to have the breasts of a mermaid, or the muscles of a merman. That is the next thing to change since you can now breathe like the fishes."