Long story short, Jaesuk wanted to prank Kwangsoo, but couldn't figure out what prank should he pull. Without much thinking, he decided to purchase some random drink which was promoted by a celebrity friend of his, which could switch your gender. Jaesuk didn't believe in such magic as he remembered reading those kinda stuffs in fairytale books when he was a toddler. 

However, Jaesuk being his silly self decided to buy it and give it to Kwangsoo. He said, "Yah, Kwangsoo, if you drink this all of your wishes would come true, " which was of course, a lie but Kwangsoo being so desperate at finding love and prosperity took the drink and thanked Jaesuk profusely. Jaesuk could only hold his laughter and try to act calm to not expose his lie. 

Nonetheless, bad news for both Jaesuk and Kwangsoo, the next day the latter transformed into a lady , and Jaesuk had to take responsibility of his action no matter what.