One week later.
That noon, Murphy was discussing a newly invented potion with Cho Chang.
"You're saying this Polyjuice Potion can't turn someone into another person's appearance, but it can change the drinker's sex?"
Murphy was stunned. "How did you make it?"
"It was actually a complete accident..." Cho Chang looked a little embarrassed. "Before, following your suggestion, we wanted to do some research on animals transforming into animals. Our test subjects were fish and mice, because they were easier to get..."
"But during one experiment, we accidentally dropped some fish eggs into the potion. Later, we added a mouse's hair to the potion and fed it to another mouse. Its appearance didn't just change—it even turned from a male mouse into a female mouse, even though the mouse that provided the hair was clearly male too..."
"We were very curious about it, so we ran many more sets of experiments before we discovered that it was those fish eggs that caused it."
"If you drink a potion with only fish eggs added, there's no reaction at all. But if tissue from a creature of the same species is added, regardless of that individual's sex, the drinker will transform into the other sex... boys become girls, girls become boys... as if they had always been that way..."
"What's strange is that the tissue from the same species seems to be used only to activate the potion. It doesn't give the drinker that individual's traits. After taking it, the mice only underwent a sex change and still retained their original characteristics, such as fur color, scars, and so on..."
"What kind of fish eggs were they?" Murphy was curious too.
"I think they were called rainbow trout?" the girl said.
Many fish had the ability to change sex. For example, a