The experiment took place the next day. In Professor Frink's laboratory, Bart and Lisa were strapped and shackled into chairs. Two metallic helmets with flashing lights and wires attached had been placed over their heads. Homer, Marge, and Maggie were watching, while Frink was giving Bart and Lisa a final briefing. "Remember," he advised them, "this is a top-secret experiment, so if anyone asks me about it, I will deny all knowledge. I recommend that you claim to be the person whose body you possess, since nobody will believe you otherwise."

"I'm so proud of my babies," Marge gushed. "They're sacrificing their bodies for science!"

"Science?" said Homer in surprise. "I thought we were getting a million dollars!"

"This won't hurt a bit," Frink assured the kids.

"Then why are these shackles necessary?" asked Lisa.

"When the transfer is complete, you may react to your new bodies in an irrational manner," said Frink. "The shackles will prevent you from hurting yourselves."

"You're trying to scare me," said Bart cockily. "It won't work."

"Are you ready?" Frink asked Bart and Lisa.

"Ready!" they replied in unison.

Frink reached up, grabbed a large handle on the wall, and started to pull it down. Bart and Lisa gritted their teeth. A few seconds passed, but nothing had changed.

"Hey, I'm still me!" Lisa observed.

"Your stupid invention doesn't work!" Bart complained.

"Invention?" said Frink. "No, I was just turning on the air conditioning. It's hot in here, ng'hoyven."

Having said that, Frink pressed a large red button on a console. Electricity flowed into the wires attached to the helmets. Bart and Lisa experienced a tingling sensation that lasted for about five seconds. When it passed, they looked down at their bodies, then at each other, and screamed with fright.

"AAAAAARGH!"

Homer and Marge beheld the transformation in amazement.

"Omigosh!" exclaimed Lisa through Bart's mouth. "My voice! That's Bart's voice! I really am Bart! The invention worked!"

"Mom, help!" wailed Bart from within Lisa's body. "I sound like a girl! I'm wearing a dress!"

"This is so freaky," said Lisa-in-Bart in wonderment. "I had no idea being a boy felt so different. It's scary, but I think I can deal with it."

Seeing that Lisa had calmed down, Frink loosened the girl-turned-boy's shackles. Bart continued to squeal in girlish terror as Lisa, Marge, and Homer gathered around her. "I want my body back! Give me my body back!"

"But if he switches us back now, we won't get the money," Lisa reminded her brother.

"There's nothing wrong with being a girl," Marge assured him. "I was a girl, and look how I turned out."

"Girly girly girly, girly girly girly!" said Homer mockingly.

"Dad, cut it out!" said Lisa.

"You can go ahead and sell Maggie for all I care!" protested the panicked Bart. "I want to be a boy again!"

Marge was taken aback. "Sell Maggie? What are you talking about?"

"I...I heard you talking about selling Maggie to the black market," said Bart. "That's why Lisa and I volunteered for the experiment."

"I don't know what you heard," said Marge, "but we would never sell Maggie. Not for a million dollars."

"A billion, maybe," added Homer.

"Then we did this for nothing!" cried Bart, nearly on the verge of tears.

"Not for nothing," Lisa corrected him. "We still get a million dollars."

"I don't want a million dollars!" Bart wailed.

"Oh, come on," said Lisa impatiently. "You'd do anything for a million dollars. You'd stand on your head for a month. You'd eat nothing but liver and onions for a month. Is being a girl for a month really so bad?"

"Yes," replied the anguished Bart. "I feel weird. It's like cooties are swarming out of every orifice in my body. And my eyelashes...they're so long...so heavy..."

"Just a minute ago you said you weren't scared," said Lisa. "Are you scared now?"

Bart clenched Lisa's fists and tried to be brave. "Yes! I mean, no! I mean...Professor, get these shackles off me so I can get out of this crummy lab and go on with my life as if nothing had happened!"

"I will," said Frink, "once I'm finished locking away all the sharp pointy things."

Once the professor had released Bart from his chair, he led the confused kids to a full-length mirror, where they viewed their new reflections in astonishment. Lisa ran her fingers through Bart's pointy hair, while Bart could only stand still and gape.

"This is amazing, Professor," said Lisa. "I'm boldly going where no girl has gone before."

"Uuurgh..." Bart groaned.

As the family drove away from the laboratory with Bart, Lisa, and Maggie in the back seat, Bart folded his arms and sulked. "Mom, Homer, can you buy me some pants?" he asked in an unenthusiastic tone. "I don't want to wear a dress."

"If you buy her pants, I won't wear them after we switch back," said Lisa.

"I'm not a 'her'," Bart protested.

"Besides," Lisa went on, "what would the kids at school say if they saw Lisa Simpson wearing pants?"

"They'd say, 'Hey, look, there goes Lisa Simpson wearing pants,'" replied Bart.

"No," said Lisa, "they'd say, 'Hey, look, there goes Bart Simpson in Lisa Simpson's body wearing pants.'"

"Ay caramba, I didn't think of that!" said Bart. "If the bullies see me like this, they'll pound me into mush!"

"Not if you pretend that you're really me," Lisa suggested.

"But how can I do that?" Bart wondered. "I mean, you're like, smart, and stuff."

"We'll just tell everybody that...that Lisa's gone bad because she feels unappreciated by her family, and Bart's straightened out because he's learned through bitter experience the unpleasantness and futility of underachievement."

"Or we could tell them that aliens have taken over our bodies. That would be more convincing."

When the Simpsons arrived at home, Bart and Lisa took up positions in front of the bathroom mirror, where they gazed in wonder at their new bodies. "The only thing that keeps me from tearing off these stupid girl clothes is the prospect of seeing myself naked," Bart groused.

"I'm your little sister," Lisa pointed out. "You've already seen me naked."

"Yeah," said Bart, "but then it was funny because it was happening to someone else."

As Marge walked in to check on the kids, Bart took off Lisa's pearl necklace and handed it to his sister. "Here's your stupid necklace," he said bitterly.

"Lisa...er, Bart, don't take off the necklace," Marge urged him. "It makes you look pretty."

"I didn't have to look pretty when I was a boy," Bart grumbled. "Why should I start now?"

"Hey, look!" said LIsa jokingly. "There goes Bart Simpson in Lisa Simpson's body without a necklace!"

"It won't work, Lis," Bart grumbled. "Take the freakin' necklace."

"If you insist." Lisa took the necklace and--to Bart's dismay--started to place it around her own neck.

"Okay, okay!" exclaimed Bart. "I'll wear the necklace!"

Once he had taken the necklace from Lisa and put it on, he stepped out of the bathroom. "If anybody needs me," he said flatly, "I'll be in my room. For the next thirty days."

"When you say 'your' room," asked Lisa, "do you mean Bart's room, or Lisa's room?"

"Oh, geez!" Bart answered sharply. "Bart's room! My room! I'm Bart!"

"But I should have your room," Lisa opinionated. "I have to wear your clothes and shoes, and they're in your room. Plus, I want to learn as much as I can about how boys live, so it only makes sense that I should stay in a boy's room, and sleep in a boy's bed, and play with a boy's toys."

"Well, if you think I want to stay in a girl's room, and sleep in a girl's bed, and play with your stupid Malibu Stacy dolls, then you can go straight to..."

"You watch your language, young lady!" Marge interrupted. "I mean, young man!"

"Please, Bart," Lisa urged. "You don't have to play with my dolls. In fact, I'd prefer you didn't."

"Oh, all right. I'm going to Lisa's room, and I won't be out for a month." With that, Bart stormed into Lisa's bedroom and sealed himself inside.

Meanwhile, Lisa wandered into Bart's room and started to examine the various toys and comic books littering the floor. One in particular caught her attention--a radio-controlled monster truck. "That thing drives me crazy," she thought. "I don't understand what Bart sees in it. Maybe it's something you have to be a boy to appreciate."

When Bart stuck his head out of Lisa's bedroom door about ten minutes later, he saw Lisa in his body with the remote control, driving the noisy truck up and down the hallway, apparently having a great time. "Hey, Lis," he requested, "I need to take a whiz. Can you show me how?"

TBC