Twilight Sparkle sat in her library, studying. She had recently returned from a lovely picnic with her best friends, taken as a break from her studies. The only real bump was that she'd forgotten to bring a ground cover to sit on as they ate, but rather than anypony going home and getting one, they just lay directly on the ground with no apparent problems, so having forgotten only bothered her in the meta sense that she felt it should have bothered her. But, with that bit of relaxation done, she felt it was time to hit the books, scrolls, and whatever other research materials she had access to that were relevant to the topic at hoof.
She hadn't gotten very far into it, however, when the door burst open.
"Twilight! We need your help!"
"Mr. and Mrs. Cake!" Twilight said. "What is it?"
"It's Pinkie!" said Mr. Cake. "She just started laughing, and she won't stop!"
Twilight quirked an eyebrow. "Well, she is supposed to be the Element of Laughter. You're sure this isn't normal?"
"Reasonably sure, Twilight," said Mrs. Cake. "I didn't think much of it at first; I thought she had just come up with a joke, but then I realized it was more like tickled laughter, and I thought she was letting the twins tickle her. But then I saw there was nopony near her, and there wasn't anything else that I could see, yet there she was, helpless with laughter."
Twilight sighed. Well, so much for research. "All right, I'll go see what I can do."
Just then, however, three young fillies came in behind the Cakes.
"Twahlight!" said Apple Bloom. "Ya gotta help! Applejack was at dinner, an' she jus' started laughin' uncontrollably! Ah ain't seen her laughin' lahk that since that tahm Trixie got holda her!"
"The same thing's happening to my sister!" said Sweetie Belle.
"Rainbow Dash, too!" said Scootaloo.
"Okay, hm," said Twilight. "Maybe you should try to bring them here...?"
"Ah guess we could do that," said Apple Bloom. "Ah'll git Big Mac ta help!"
"Okay, good!" said Twilight.
Just then, a white rabbit hopped in, and up to her.
"Angel?" asked Twilight. "What is it?"
The lapine held up a sign depicting Fluttershy laughing on the left side, and on the right was a wiggling feather and a question mark. Twilight wasn't really an expert at deciphering Angel's pictures, but given what she'd already been told...
"Change of plan, everyone," said Twilight. "Bring them to Fluttershy's!"
And with that, she disappeared, leaving the others to gather her friends.
*~*~*
Within a few minutes, Twilight arrived at Fluttershy's cottage with Zecora. It didn't take long to find Fluttershy; her laughter ranged from adorable giggling to loud laughter, looping back around to silent open-mouthed laughter.
"I've tried See Invisible, checked for magical auras... nothing," said Twilight. "Do you have any ideas?"
"Tickled she is; no tickler found. Has her belly touched the ground?" asked Zecora.
"I guess it had to, when we had our picnic earlier today," said Twilight.
Just then, the increasing sound of laughter announced the approach of a cart, pulled by Big Mac and ridden by the Cutie Mark Crusaders, the Cakes, Angel, and four laughing Elements. Once it stopped Twilight quickly started carefully laying them on the ground one at a time as Apple Bloom unhitched Big Mac, everybody else disembarked, and Zecora pulled out a magnifying glass and started investigating Fluttershy's belly. The air was filled with laughter and shouts of "Oh no!", "That tickles!", "I'm ticklish!", "Make it stop!", "Please!" and other such phrases.
"I see what your magic can't," announced Zecora after a few seconds, "hairs of the tickling nettle plant."
"...Urtickle dioica!" said Twilight. "I thought those plants looked familiar. 'Leaves of half / makes you laugh.' Why didn't I remember that then?"
"It's in our nature to forget," said Zecora, "but a treatment must be set. These hairs must be brushed away, and the tickles you'll allay. But a brush won't cut their tethers; stroke them back and forth with feathers. Birds' will work, but hard to get; better are your lovely set."
"...From my wings?" asked Twilight. "Well, okay, here goes! Everybody take one!"
Twilight started telekinetically removing loose-seeming feathers from her wings, and each of the non-Elements present took a feather and went over to somepony. The Cakes used their feathers on Pinkie Pie, Applejack was treated by her siblings, Sweetie Belle got Rarity, Scootaloo got Rainbow Dash, and Angel stroked Fluttershy's belly with the feather he picked up. Twilight looked around, but found no spare feathers.
"Huh," said Twilight. "Oh well, looks like everypony's covered."
Of course, given that the treatment alone was wiggling feathers on ticklish tummies, the initial effect was to make the tickling more intense than before, and this was reflected in the laughter of the five ponies; however, as hairs were removed by the tickly motion, the laughter started to decrease, indicating that it was, indeed, working. When a pony's laughter seemed to be at normal levels, the one or ones treating her would stop, have the patient point out where it still tickled, and, if it did, continue treatment. Once no spots were found, the treatment was over, and the pony simply rested and caught her breath. After about a minute, all ponies were treated. The little sisters -- real and unofficial -- snuggled up to their elders, as did Angel with Fluttershy, while Mrs. Cake gave Pinkie a soothing bellyrub.
"Now the feathers you must burn," said Zecora, "the smell will surely help you learn."
Twilight nodded, remembering the last time she smelled burning pegasus feathers. Thankfully, the pegasus attached to them was otherwise fine, but she couldn't get to work for a while and had to use some "sick days".
"Nayow," said Applejack, "what in the hay was that? What happened t' us?"
"Yeah!" said Pinkie. "It can be pretty fun to be tickled by your friends, but when you don't know what's doing it, and it won't stop ever even if you say 'pretty please'? What's up with that?"
"Oh, I..." said Twilight, blushing. "I think you all must have lay in some tickling nettles when we had our picnic, eheh. Sorry, guys, if I had remembered the blanket or went back for it anyway, you would've been protected."
"Aw, that's okay, Twahlaht," said Applejack. "Everypony makes mistakes. Although..."
"Although...?" said Twilight, not sure she liked how Applejack said that.
"..yer the one who made it, an' nothin' happened to you," Applejack pointed out. "Now, if'n it were somethin' really bad, t'ain't no way Ah'd suggest somethin' lahk this, but..."
"...I guess it's only fair," sighed Twilight. "All right, go ahead." She lay on her back, leaving her belly vulnerable.
"Rainbow? Fluttershah?" said Applejack. "If'n y'all would do the honors...?"
"On it!" said Rainbow, coming up beside her.
"Oh dear," said Fluttershy, approaching more slowly. "Please, let's not be too hard on her...?"
"Don't worry," said Rainbow Dash, "it'll just be a taste of what we've had to go through!"
"O-okay," said Fluttershy.
With that, the two pegasi used their wings to tickle Twilight Sparkle's belly.
"Oh no!" said Twilight, and started laughing. "Ah ha ha ha ha ha ha ha ha ha hee hee hee ha ha ha ha ha ha ha ha ha ha ha ha ha ha! That tickles! Haa ha-ha ha-ha ha ha ha ha ha ha ha ha ha ha ha ha ha! I'm ticklish! Ah ha ha ha ha ha ha ha ha ha ha ha ha!"
As the tickling continued, Zecora quietly slipped away into the forest.
Author's Note
Did I just make her more obsessive? 
