"I wish..."
Wait. Jon thought. Maybe it's a bit premature to be wishing for something else. I mean, I haven't even tried out my previous wish yet. And how do I do that exactly?
"I wish I knew how I could be a mermaid." Jon said, finishing the wish.
And then he knew. All he had to do was concentrate on the concept of the mermaid, and he would become one. Promptly, he began to do just that. Engrossed with imagining what it would be like to be immersed in water, his legs replaced with a fishy tail, he overlooked an important error on his part.
Mermaids were female.