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what you do to me

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"There's a package for you," Derek says, squinting at the shipping label. "It's from Japan."

"Awesome!" Stiles's voice is tinny, distorted by the shitty reception he always gets in the dorms. "Don't open it. It's a surprise."

Derek sniffs the package. It smells like people he doesn't know and metal, nothing interesting.

"Stop sniffing the mail, that's weird," Stiles says.

"I'm not sniffing the mail," Derek says.

Stiles sighs. "I'll be at the house at ten. Be in your room, naked, lights off. Leave the package on the table by the door."

Oh, it's that kind of surprise.

Derek always meant to tell Stiles, he always means to tell Stiles, but somehow, he never gets that far. The one time he tried to talk about Kate, Stiles held his hand and gave him that intense focus Stiles only gives to things he—anyway, Stiles told Derek that he never had to talk about if he didn't want to, that was okay, Stiles understood. Only he didn't understand, and now Derek is afraid to talk about anything. Mostly because there isn't that much to tell.

The first time they had sex, it was the full moon, and things went very quickly from Derek talking to Stiles as Stiles flipped through the grimoire that Derek had brought down to Davis to Stiles rimming Derek on the bed, cupping his balls until Derek came all over the sheets, half wolfed-out, dick untouched, and deeply confused.

"Oh, I just had a dental dam lying around, they gave out a little goody bag during the safe-sex education thing," Stiles said breathlessly while Derek jerked him off. "Don't look so shocked. I'm not licking your butt without one of those unless there's some really thorough showering beforehand, dude."

Derek was going to explain, but then Stiles came in his hand, beautifully surrendering, shuddering, and it was too much to deal with, all in one go.

After that, they went to a diner and got milkshakes. Stiles provided running commentary on the grimoire as he paged through it, absently sucking down his chocolate-and-banana side-by-side when he paused for thought. He didn't talk about the sex at all, so Derek didn't say anything. His strawberry milkshake was okay, although Stiles stole the cherry.

Derek falls asleep on his bed, waiting. He doesn't do that much, even now that the house has been rebuilt for two years and untroubled for a solid eighteen months, just falling asleep where he is without checking the doors and the windows and the runes Stiles carved into the frames, which each have a spot where anyone can rest their hand to make sure they're still active. It's a different kind of magic from the one that thrums in Derek's blood, but Derek can feel the house sing when Stiles comes in the door, satisfied, drinking up the spark that hovers around him like a nourishing flame.

The house doesn't wake Derek this time, only Stiles's fingers brushing against Derek's shoulder and his breath warm against the shell of Derek's ear. "Hey, you were out cold," Stiles says when Derek murmurs an acknowledgement. "You still want to—we can do this later."

Derek wants to say yes, wants Stiles to come curl up beneath the quilt on his bed so Derek can touch him and hold him until he smells like home again, but he remembers how eager Stiles sounded on the phone, how delighted to give Derek a gift, like everything else he gives Derek isn't something Derek can never repay. "I want to," he says, letting Stiles push him over until he's lying on his stomach. "Yes."

"So this thing, it's really cool." The tape on the package squeaks and gives up the ghost as Stiles cuts through it with the swiss army knife he keeps on his keychain, keys jangling in accompaniment. Stiles likes multipurpose tools; he never sees the point in anything with only one use, whether it's a grapefruit spoon or an underwater breathing spell. "It was kind of expensive, and I had to wait a while for it—it was a Kickstarter thing, like those edible cups I got Scott?—but I thought, well, you can always hear mine, you know?"

Stiles fumbles with plastic wrap, pulls some things out of his pocket that rattle dully in his palm—batteries, probably—and there's the slide of plastic against plastic, the soft sound that accompanies the depression of springs, and a sharp click. Another minute of rustling later, Stiles drops the box down next to the bed and kicks it over by the bookshelf where neither of them will trip over it in the morning.

"I hope you like it," he says, shifting on the bed, and he's clipping something around Derek's waist—an elastic belt, but there's a small plastic box with cool metal dots that rests at the small of his back, and something else, soft, that brushes against Derek's ass. Derek inhales sharply. "I just—I thought." The flick of a switch. "I wanted to know."

The soft thing starts moving, swinging back and forth across his ass. It—it tickles a little, it feels weird, the whole thing's so disorienting that Derek doesn't put it together immediately.

"It's a tail," Stiles says. "It wags. Silly, right? Except it's a special tail—it responds to your heartbeat. It'll wag slower when you're calm, faster when your heart speeds up."

"My heartbeat," Derek says.

Stiles is leaning back, now, reaching for something on the shelf where Derek keeps the lube shoved behind his copy of Dune. The cap opens, liquid drips—onto Stiles's fingers, Derek's seen him do it a dozen times, but it's still—and the tag wags faster. "Fuck, that's hot, this is… oh my god, Derek, if I come in my pants, you're just going to have to deal, this is possibly the hottest thing I have ever seen in my life, and I'm including that time last summer when Lydia was making out with Erica and they were both wearing bikinis and you were just sleeping in the lawn chair next to them—"

The lube is still cool, although it's started to warm on Stiles's fingers, and Derek squirms a little when the first one presses into him, slow and deliberate. His tail slows a little. Stiles doesn't usually start with one finger unless he's planning to drag it out, to tease Derek until he comes and comes and cries out, helpless, wordless, undone.

"—I got you a black tail," Stiles says. "Actually, I got you all the tails—there's four colors, right, and I thought, why not, maybe you'd like it, let's go for a little variety—but this one looks like when you're the alpha, it matches, right—"

Stiles doesn't go right for that spot that makes Derek twitch until he warms up to it. His finger skirts it, pulls back, goes in again, twice, three times, four, before he adds another finger. Derek wants to fuck back onto Stiles's hand, he always does, even this soon, but the one time he tried Stiles braced his hand against Derek's hip and said, "Let me, okay?" So Derek does. Stiles seems to know what he's doing.

"—you always, and it's not even when you do it in bed that does it to me, it's when I have to explain to Jackson that he's a fucking idiot and you just touch my shoulder, you—and that time when Scott and I got kidnapped by vampires and you found us in, like, five seconds—"

It felt a lot longer than that.

"—and Allison linked me to the page when I was feeling down, because there's this ridiculous promotional video, right, and tails, you know, it's fucking hilarious, except then I started thinking about it—"

Stiles has three fingers in his ass now, and the tail's wagging hard, it's probably brushing against Stiles's hand, sticky with lube, although it doesn't feel wet when it swishes against Derek's thighs. Hopefully it's washable.

"—I wanted to know, just, how you feel, when I'm fucking you, when I'm—"

Derek forgets all about the tail when Stiles puts his hand on Derek's dick, starts stroking him, firm and steady, the way Derek likes it. The heartbeat thing is stupid—Stiles always knows what Derek likes, knows just what to do to him, better than Derek knows himself. In bed, between the two of them, Derek's just fumbling around, hoping he gets it right, hoping he doesn't fuck it up too bad before Stiles figures everything out.

"Fucking christ," Stiles says, "what you do to me."

Usually, Stiles stays the night after, although he never stays the whole weekend. Derek never asks him. The sex is good, it's really good, but Derek doesn't actually have much of a standard of comparison. There was Kate, there was a year where he didn't even jerk off, just woke up uncomfortable and sticky in the night every now and then, then there was his hand up until there was Stiles, licking into him, getting right into the places where Derek never let anyone touch, places he never breached on his own. Whenever Stiles asks him what he wants, he says, "This is good," and, "Yeah, I liked that," because he always does, he likes whatever Stiles does with him.

He likes Stiles.

Someday, Derek will probably tell him that.