Not as advertised

"Mabel?" Dipper said slowly.

"Yeah, Bro-Bro," she replied, looking up from her knitting.

"The journal has a section on mermaids," he said.

"What about mermen?" Mabel asked.

"There aren't any," Dipper said.

"What?" Mabel stopped knitting and came over to read the journal entry. "Female only species, lonely sailors and beach goers, deep voices for speaking underwater…"

"We really should have questioned why a merman would have 'merman' in his name," Dipper said.

"Why would he, I mean she, lie to me?" Mabel asked, upset.

"Hmm," Dipper said as he considered it, clicking his pen as was his habit.

Mabel waited, knowing dipper was usually pretty good about figuring these things out. "Well?" she asked, after a couple of minutes went by and he hadn't said anything, her patience exhausted.

"I think I know," Dipper admitted. "Go get a pair of shorts and strip down until that's all you have on."

"What? No," Mabel said with a frown.

"Why not?" Dipper asked reasonably.

"Because I'm not going to wander around topless even if it's just us here, someone could come in."

"What if we disguise you as a boy?" Dipper asked. "We could call you Marvin."

"That'd still be really embarrassing," Mabel said before admitting, "but I'm intrigued."

"Imagine if you were trapped in a public place with no shirt and had the voice to pull it off," Dipper said, seeing that trying to lead Mabel towards figuring it out herself was backfiring.

"Ok, I can see why she pretended to be a boy," Mabel admitted, "but why didn't she tell me later?"

"You had just rescued her and seemed really caught up in it all," Dipper said. "She didn't want to disappoint you and she probably really liked you."

"Really liked me?" Mabel squeaked.

"There are no mermen and they spend most of their time in the depths of the ocean," Dipper hinted.

"Oh!" Mabel said, blushing.

"You were her knight in shining armor," Dipper said with a proud smile.

"And yet you got more lip action with her than I did," Mable said. "You made out with my girlfriend!" Mabel laughed.

Dipper grinned.

"Now lock the door and pass me some shorts, I'm going to send 'Mermando' some pictures of her boyfriend Marvin!"

"What?"

"Should I wear a fake mustache?"

"No!"

"Ohhh, we could dress you as bare chested werewolf and I'll be a young Count Dracula, that way she gets pictures of both her boyfriends!"

"Mabel!" Dipper groaned, suddenly suspicious of where Grunkle Stan had gotten the idea for the pre-teen wolfboy costume he made him wear.

Typing by: Stephenopolos