You and Riko have been enemies ever since elementary school. You guys despised each other.

The two of you wouldn't stop fighting and bickering with each other. Whenever you would pass by Riko, she would either start whispering to her friend, or she would try and trip you. Or even both.

...

One day, you and Riko were paired up by the teacher for a project. The two of you had to make a potion of any kind and present it to the class in a few weeks.

Riko has been messing with the potions during the process, as well as constantly playing around. You got annoyed by her actions, and snatched the potion she was holding away from her. Riko tried to grab the beaker again, but the liquid spilled all over the two of you, making you even more irritated.

Suddenly, you felt... weird. When you look up at Riko, you saw... yourself!?

"Dang... why do I look so pretty...she says, smirking as her eyes linger on you. "Wait a minute- why do I look like a rat??" she looks at a nearby mirror.

It appears that you both have swapped bodies from the potion.