When Dipper discovers a hidden room inside the Mystery Shack, he is happy to claim it as his own space, but there is one problem- Mabel wants the room too. The duo compete to win the room with a contest that gets even trickier when they learn the room's carpet has electric body swapping properties.
Dipper and Mabel gleefully play a game of "attic stuff mini-golf," goofing around together before Mabel's alarm clock goes off. Though Dipper is initially disappointed to find out that his sister is spending another night with Candy and Grenda, he becomes horrified upon learning that Mabel's friends will actually be staying at the Mystery Shack. The girls engage in various sleepover activities (e.g. talking about boys, and giving each other makeovers), and Dipper grows increasingly annoyed to the point of leaving the room to find another place to sleep, considering Soos' horrible break room, which is just an electrical panel with hardly enough room for a person to fit, until ultimately deciding to sleep outside, despite a wolf biting and gnawing on his leg, and an owl trying to eat his tongue.
The next morning, Mabel and her friends wake up and briefly reflect on their memories of the previous night, despite much confusion. After Candy and Grenda left, Dipper comes back inside and confronts Mabel about her frequent sleepovers, and the two come to the agreement that they need separate bedrooms after fighting. Though Stan says that there is not enough room, the family soon learns of Soos' discovery of a hidden room while cleaning, which both twins come to want. Stan decides to take advantage of the situation by having the twin who sucks up to him the most get the room, and each task earns them "Suck-up Points" and they engage in various chores for their uncle like tying his shoes, putting new shingles on the roof in searing heat, etc. Down in the room, Dipper relaxes, feeling confident that he will definitly get the room, and rubs his socks on the floor, building up static electricity. The twins begin to argue again. Dipper then hits his hand on Mabel's hand and the static electricity causes them to swap bodies, and they quickly start to freak out.
After the two figure out that the experimental carpet in the room swapped them while Dipper conducted static electricity, they also realize that they can sabotage their uncle's view of each other while in each other's bodies. Mabel (Dipper) makes Stan a sandwich made out of rocks while Dipper (Mabel) sings "Breaking stuff is so much fun, I am Dipper and I stink," while Stan, not knowing what happened, is confused. Waddles rolls around on the electron carpet while Soos cleans and comes to pet him, thereby swapping them, fulfilling Soos's wish of wanting to be a pig. Meanwhile, Candy and Grenda arrive to have yet another sleepover with whom they think is Mabel (actually Dipper), and reject Dipper (Mabel)'s request to talk to her (him). Mabel (Dipper) tries to get away, but Grenda suggests that they should read a romance novel about a werewolf. Dipper (Mabel) spies on them through the keyhole, trying to listen to the book reading, when Stan discovers her and, thinking Dipper is spying on the girls, decides that they need to have a talk about the birds and the bees. Downtown, Soos enjoys being Waddles, and is rolling in mud until Old Man McGucket arrives to eat him, while Sheriff Blubs and Deputy Durland read their horoscopes from their car.
After Stan's finishes reading a book titled "Why Am I Sweaty?" to her, Mabel is scarred and tries to further sabotage her brother, but ends up winning him the room instead, after calling Stan a stupid old jerk. Dipper is forced to sit through girly activities with Mabel's friends, and the chase between McGucket and Soos continues, much to the horror of all three. Next, Mabel informs Dipper that he won the room, but locks herself in there when she realizes her brother cannot get in without the key. Dipper enlists Candy's and Grenda's help in breaking in, by giving Dipper (Mabel) a makeover, but the four of them start swapping bodies left and right after Candy and Grenda learn about the carpet's properties. Meanwhile, Waddles unintentionally gets Soos a raise and wins the attention of a female passerby looking for directions.
As more and more people show up in the secret room, chaotic body swapping ensues. The twins make sure they're themselves and then run to the attic to continue to fight over the key of the basement bedroom, but soon learn that neither wanted to move out in the first place and come to realize their motives in the situation. After everyone is put back in his or her own flesh, Soos remarks that he learned being a pig was not as great as he thought it would be, and discovers that Waddles has gained him a beautiful fiancée and a raise. Dipper moves into his new room and has Stan dispose of the Electron Carpet. Dipper begins to say "good night" to Mabel, when he realizes she isn't there, and vice versa. Dipper gives Soos the bedroom, to replace his horrible closet break room, and moves back with Mabel. Meanwhile a confused Soos talks to his apparent fiancée about not knowing anything, and asks her if they can go back to kissing.
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- Mabel awakes from her slumber party with the Sev'ral Timez movie on her face, referencing the future appearance of the boy band in "Boyz Crazy."
- The pair of eyeglasses Stan picks up in the new room are the ones that young Stan was wearing when he briefly appeared in the "The Time Traveler's Pig", and the one that the Author is wearing.
- Robbie's Explosion muffin is seen on the water tower.
- The Cloning copy machine from "Double Dipper" makes another cameo appearance in Stan's office.
- Viewership: This episode was watched by 3.36 million viewers on its premiere night.
- The name of the episode refers to the Latin phrase "Carpe Diem" which in Latin means, "Seize the Day".
- This episode aired as part of Freaky Freakend on Disney Channel. A promotional stunt wherein eight episodes from various shows have body switching episodes over the course of a weekend.
- The omelet that Mabel made for Stan that looks like his face has his original fez design.
- The bobble-heads that Waddles (in Soos' body) knocks over in the gift shop show the new fez design.
- When Stan is looking at the glasses, the date circled on the calendar is July 4, 1982.
- While this date on the calendar was a Thursday, July 4, 1982 was actually a Sunday.
- Stan is later briefly seen sitting in his chair, holding the glasses and looking at them with a frown.
- When this episode was put on iTunes, the episode itself was "Boyz Crazy" by mistake.
- It is unclear if other characters hear the voice the audience hears, or the one associated with that body.
- It is revealed in the episode that Dipper is going through puberty and is feeling awkward and sweaty as a result.
- The date on the calender, July 4th, 1982, is Independence Day. 1982 is also the year Stan started practicing a coin trick that he still hasn't been able to master to this day.
- 618 appears again on the block that one of the babies chews on in the show Baby Fights.
- There is a cryptogram at the bottom of a page in Why Am I Sweaty? that uses the Caesar cypher. It says, "SXEHUWB LV WKH JUHDWHVW PBVWHUB RI DOO DOVR: JR RXWVLGH DQG PDNH IULHQGV." Once decoded it says, "PUBERTY IS THE GREATEST MYSTERY OF ALL ALSO: GO OUTSIDE AND MAKE FRIENDS."
- The cryptogram in the end credits reads "2-21-20 23-8-15 19-20-15-12-5 20-8-5 3-1-16-5-18-19?" Once decoded it reads: "BUT WHO STOLE THE CAPERS?", referring to the book on Dipper's summer reading list, The Sibling Brothers and the Case of the Caper-Case Caper.
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