In K-pop, it's not so rare to see cross-dressing, but as common as it is to see guys imitating their female counterparts, rarely will girls dress up as boys.

Some girls, like f(x)'s Amber, will have boyish concepts, but very few female idols dress up and perform stages dressed as male idols. In contrast, there are numerous cases where members of boy bands will get up and happily perform girl groups' songs.

 For the 400th episode of Music Core, several members of four popular boy bands teamed up to create a parody of Girl's Day, Boy's Day.

Nu'est's Ren, BTOB's Minhyuk, VIXX's Hongbin, and A-JAX's Seungjin wore almost identical outfits to what Girl's Day wears on stage for performances of "Something" and danced the sexy dance that gained Girl's Day a lot of attention.

But Boy's Day is only the most recent parody group.

Parody groups often appear at year-end award shows, holiday specials or even at their own concerts, and fans usually accept it as the ultimate fan service.

Sometimes they merely perform the songs, but, as in the case of Boy's Day, they may even mimic the look of the girl group.

The boy band members who are usually picked for the parodies are either the most delicate looking in the group, or else the member who is best known for imitating girl group dances on variety shows. Some parodies are only dance performances, while other parodies actually even cover the song.

On MBC's Star Dance Battle, 2AM parodied miss A's "Bad Girl Good Girl" and became miss AM, performing "Bad Boy Good Boy." They wore tight-fitting outfits with skirts, and even had their hair styled in the same way that miss A had it in while promoting the song.

2AM has done several girl group parodies at their concerts, and Jo Kwon is pretty well known for his girl group "kkap" style of dancing.

B2ST's Kikwang, ZE:A's Kwanghee, and SHINee's Key are also kings of girl group dances, as shown by their cover of Orange Caramel's "Magic Boy."

Although the three didn't wear the colorful dresses, Kikwang and Kwanghee wore mouse ears, and they all wore polka dots to imitate the girl group's look.

Even though it was only two people and not a true girl group parody, INFINITE's Troublemaker parody gained a lot of attention when Sungyeol transformed into 4Minute's HyunA to perform "Troublemaker" with maknae Sungjong at their Second Invasion Concert.

It was a bit of a turn around for INFINITE, where Sungjong is generally considered to be the best girl group dancer, so fans were pleased to see Sungyeol take on HyunA's role.

The Girls' Generation song "Gee" created an amazing craze in Korea, and Boys' Generation is one of the most famous covers.

Instead of the nine members of Girls' Generation, this group was made up of Super Junior's Shindong, Eunhyuk, Yesung, and Sungmin, 2AM's Jo Kwon, SHINee's Key, and 2PM members Taecyeon, Nichkhun, and former leader Jaebeom (now Jay Park.)

The nonet was dressed in colorful skinny jeans and white t-shirts like Girls' Generation, and performed with a lot of enthusiasm.

What is your favorite K-Pop girl group parody? Let us know in the comments section below!