Author note: Hello! I'm italian, so there will be a lot of mistakes probably... Any advice is well accepted since I'm trying to improve.
Hope this story will tickle your fancy ;)

Stiles and Derek were at the werewolf's house, cuddling a bit on the couch. Stiles was laying with his head on Derek's chest while the werewolf caressed his back, from the back of his neck to the lower back making Stiles almost moan.

After a while, anyway, Derek's hands started to wander closer and closer to his sides and everytime they reached that spot Stiles started to squirm an suppress giggles.

"Derehehek!"

"Yes, Stiles?" he asked innocently, scratching his ribs with nonchalance.

"Ah! Ahahahahaha!Stopstoopstoooohohohohpppp!" The human was trashing around, unable to stop laughing and to free himself.

"Stop doing what, exactly?"

"Fuuhuhuhuhuhckkkkk youhuhuhuh! Dahahahamn!"

"Is that what I should stop doing? Nah…"
When his hands moved higher and higher getting closer to Stiles' armpits he was completely hysterical.

After a few more scratches Derek stopped, a wide grin on his face. Stiles was willing for revenge, so he poked his lover in the sides.
Derek looked at him, unimpressed.
Stiles tried to scratch his stomach, but again he gained no reaction.
"Stiles, I'm not ticklish."

The human was skeptical "You're not ticklish?" repeated, not buying it.
Derek shuggered "I'm not."

"Impossible, everyone is!" the boy'd mind was going "Scott is still ticklish even if he's a werewolf so it's not that… You are lying. I want to test it."

"What?"

"I want to test it."
Derek groaned and leaned back on the sofa, his arms up and a boring look in his eyes "Be fast at least, I'm hungry."

Stiles placed himself next to Derek, who was looking at him quite amused.
The boy moved his hands towards Derek's ribs while staring at him with suspicious, ready to catch even the lightest glimpse of ticklishness.
Derek seemed completely relaxed, though, and even when his hands made their way scratching a little the area just above his hips he didn't move. Stubborn, Stiles hands moved higher, spidering and tickling every inch of skin avaiable in their way to the werewolf's armpits. Stiles was sure that at least that spot would have made him crack up, but Derek kept watching his pale hands with a blank expression.

"Oh, come on…" Stiles snorted, disappointed. He couldn't even think of someone tickling his armpits without squirming, how could that grumpy grump be immune?

He tried every spot crossed his mind: neck, ears, knees, belly, lower back… nothing. Derek just lied there, following his hands with his bored gaze.

"Are you done?"

"No way! Everybody is ticklish, you just have to find the right spot…"

But after ten more minutes he was just going to give up. He was really disappointed, at that point. The image of Derek laughing his head off, the idea of being able to actually make him smile, laugh, maybe even squirm beneath him had been so appealing. He would have loved to be the one in charge, for once, and being the only one to know Derek's weakness like he knew his. He had even tried to imagine how Derek's laugh would have sounded. High pitched? Low? Silent? Would he have trashed around or would he have tried to stay still and be stoic?

He was so deep in these thoughts that when Derek's hands squeezed his sides he almost flew to the wall "Nohoh-no! Deehehehrek!" he laughed squirming in the warewolf lap, trying to stop the big tortourous hands that were tormenting his sides.

Derek stopped, quite amused "Can we eat, now?"

But Stiles' mind was flying "No. Just one more thing, please. My experiment isn't finished!"

"Stiles…"

"Close your eyes." he ordered, stubbornly.

"Why?"

"Last try, I swear. Close them."
Derek obbedied with a grunt.

"Arms up. Don't open your eyes, sourwolf."
The sourwolf held his arms behind his head, sighing.

Stiles took his time to look at his relaxed expression. Derek looked even peaceful. Who would have guessed that the grumpy werewolf would have ever been that calm because of Stiles' presence, and not in spite of it?
"Stiles?"

"Shh."
He counted up to ten, and then, with no anticipation, he attacked.

He squeezed Derek's sides out of the blue, and this time, caught completely off guard, Derek flinched.
A-ha!
With just the index of his hands Stiles proceeded to poke a few spots here and there in Derek's body: just a poke here in the belly, now another one here on the left side, a little scratch over his ribs… And Derek flinched at every touch, to his own surprise. At each touch his breath would break, like his sensivity was increasing more and more.

When Stiles' hands traced Derek's biceps and then flew at his armpits, Derek's compsure was gone and started squirming on the couch.

"S-Stiles!" he exlaimed opening his eyes, astonished by his body's reaction.

"Not ticklish, uh? Are you sure?" Stiles kept tickling the hollows of his underarms and Derek completely lost it: he started laughing in a few seconds: a deep, wonderful laugh. His mouth wide open, his head titling back and forth, his eyes shut. Stiles thought he was simply beautiful.

"Shuhuhut uhuhuhpppp! Ahahahahaha stoohohohohp! I'm-m gohohoohnnahahah kihihihll you!"

Very impressive, try again this time without giggling. Stiles was smiling almost as much as Derek. Pinching the flesh of his abdomen, running his fingers up and down his sides, scratching every single inch possible, he was havin the time of his life. Derek's laugh grew wilder and wilder, sounding a little higher than the usual: it was an endearing sound for Stiles, who was simply amazed.
What was even more amazing, though, was that Derek wasn't trying to stop him His hands were closed in completely useless fists on the couch. Moreover, he could have easily thrown him away with a push, but he just didn't.

"Stiihihihihles GOOHOHOHOHDDDAHAHAMN! STOHOHOHOHOHPPPIHIHIHIHT! COHOHOHOMOOOHOHOHOHOHOHN!"

"Mmh, ask nicely next time. Are you ticklish here? And… here? Oh, what about here?" asked laughing himself. His taunting seemed to affect Derek a lot since he laughed even harder everytime Stiles provoked him.
The human found a super ticklish spot on the werewolf hips and tortured it for a while making Derek go almost histerycal.

Derek, in the meantime, was shocked. He hadn't been tickled in ages, and it… tickled like hell. Its was almost unbearable, he couldn't even think, he was just too busy laughing and laughing at that tickling sensation all over him.
He had had much fun tickling Stiles, who was almost too ticklish for his own good, and now that tables were turned… Derek was enjoying much more than he could have ever imagined.

If it hadn't been Stiles, he knew, he would have never allowed any of it. But it was Stiles, it was him on top of his body, exploring and touching him everywere, making him howl with laughter.

"STIIHIHIHIHIHLEEHEHHEEHEHEHEHS EEHEHEHENOUUHUHUHG! NOOOHOHOHOHOHOH NOHOH!"
"No? No..what? Like, "no, don't stop"?" he teased squeezing his knees and making Derek almost jump to the ceiling.

Derek shook his head frantically, his hands flying everywhere.
When Stiles finally stopped Derek was too weak to do anything else but lay on the couch with heavy breath.
"So… You are ticklish, Sourwolf."
"Shut up… Did you enjoy yourself?" he asked, exhausted.

"Quite a bit, yes."
A smirk appeared on his face "So you won't mind if I have some fun myself now, will you?"

Stiles squeaked in panic, jumping on his feet with his eyes wide open. He backed off with his hands high "W-wait, come on, aren't you hungry? Let's eat, uhm?"

"I'm not that hungry anymore, you know?"
Stiles run out in the forest but ten seconds later he was squirming like a mad on the ground while Derek tickled the hell out of him.