Stiles first hears about it in the library, when Isaac and Scott are whispering and giggling and pointedly not studying for their last finals in two weeks. Which, it's not like they needed to graduate anyway what with the superpowers that could translate to pretty lucrative careers minus all the book learning Stiles has resigned himself to, but it's still annoying.
"Can you at least pretend to study when I'm over here cramming?" Stiles grumbles, re-highlighting the same sentence and blinking at the page. He's itching for some Adderall, but Scott gave him accusatory looks last week when he somehow knew Stiles took more than his prescription. He resists the temptation.
Scott and Isaac are grinning from ear to ear and looking like a pair of freshman girls with a secret. Stiles's brain is done with AP chem anyway. He closes the heavy textbook decisively.
"What?" Stiles asks, looking between them slowly.
"It's pack stuff," Isaac whispers needlessly. Stiles looks around, but no one's even close to within range of hearing. They're huddled in a corner of the library that's set apart from the other study areas.
"Ok," Stiles drawls, pointedly looking around them.
Scott covers his mouth and laughs, which sends Isaac into a fit of laughter. Stiles waits not so patiently for them to grow up and quiet down.
"Dude, just get closer, ok? It's... it's not something you want other people to overhear."
"Jesus," Stiles mumbles as he leans over the table, too curious now to resist.
Isaac leans in too, conspiratorially, and Scott follows on his elbows to join them huddled in the middle of the table.
"You know that girl I met at the pack mixer Derek dragged us to last week? Cindy? Yeah, she says that in her pack, and in a lot of packs, they have... like..."
"I don't think it's true. I think she was pulling your leg, man," Scott interrupts, looking a little guilty. He should, the gossip queen.
"Have a what?" Stiles presses with exasperation. "Should we have it too?"
"Oh, dude," Isaac dissolves into laughter again. Stiles flicks him in the nose this time, which is met with growl and a flash of yellow. "Ok, ok... the alphas have, like, one pack member... sometimes two... that they... you know."
"Can we all be big boys about this?" Stiles rolls his eyes. "That they... what?"
"They have a sex-with-no-strings kind of thing going on with one or two pack mates. She made it sound like no big deal, like standard practice. It's supposedly a normal part of pack dynamic," Scott clarifies quickly. Stiles watches his ears turn red while he talks and Stiles knows immediately that they were laughing partly out of embarrassment, thinking about who would fill that role in their own pack.
"That sounds..." Stiles wants to protest that it seems like an abuse of power, but then he starts to think about what that would like in their own pack too. Stiles first considers who Isaac and Scott might assume would fill that role, then how it's probably not who Stiles would like.
A pleasant burn starts in Stiles's stomach that creeps down his spine to his groin.
"It doesn't matter anyway because Derek's never brought it up or tried to... has he?" Scott starts to dismiss the idea before tilting to check with Isaac. Isaac shakes his head, sobering a little.
"Does that even sound like something Derek would ask for?" Isaac points out. Scott frowns and shrugs. Over the years he's come around to Derek as the alpha and the the wolf with most experience. He's settled as much as could be expected into following Derek's lead, which means he still actively rebels once a month. Derek seems to take it in stride now, though, like it's just part of their routine. Stiles thinks their relationship is greatly improved.
"Hey, Derek wanted us to learn about other packs. That's what he said when he forced us to go to that mixer," Scott points out. Isaac's laughing quietly again.
Stiles's stomach is still tingling, his mind calmer than usual.
Derek's looking at him like he just asked if he could, like, touch his hair or drive his car or some equally insane suggestion way above the level of insanity of Stiles's actual suggestion (in Stiles's opinion).
"Where the hell did you..." Derek starts, his eyes narrowing. Stiles knows he's listening to his heartbeat.
"It's because we were too young, isn't it? You started a pack with a bunch of teenagers. You couldn't."
Derek's quiet in a way that tells him he's right. Stiles licks his lips, his eyes traveling over Derek's face without his permission.
"It has to be freely given, I can't ask for it," Derek says after another moment of silence. "It's not like a... right." He sounds stiff in that way that's uniquely Derek.
Derek leans back on his sleek couch, his arms folding over his chest. Stiles sets his backpack down on the floor next to the kitchen counter. The loft is filled with soft light at this time of the day. It makes its sharp planes look a little more welcoming than usual.
"Well, it's an option," Stiles shrugs, going for casual. His heartbeat might say otherwise. Derek tilts his head, his eyes still narrowed as he studies Stiles.
Stiles can see the inside of his jeans from the way they're lumped on the floor, crinkled and folded at odd angles. He squeezes his eyes closed to another grunt from Derek as a hand cards up his chest to hold securely to his collarbone, fingers warm as they spread over his skin.
Stiles lets a moan slip as Derek rocks into him again; he loses his grip on the couch arm and stumbles onto one elbow to absorb the thrust. He can feel Derek's forehead press into the middle of his shoulders and he closes his eyes again to savor the sensation, trying to memorize the sound of Derek in pleasure.
The thing is, Stiles has waited years. He's watched, and wanted, and cried into his pillow at night as he stroked himself to the memory of Derek's voice when it reaches that low, commanding timbre. He can't believe he's getting this, and he's not about to fuck it up.
Stiles knows what to expect, he's heard about it from Scott and Isaac and even Lydia at one point, but the sensation of it is another thing entirely from the descriptions. His breath catches as he feels the pressure in his ass, feels Derek tighten up behind him, the lines of muscle rigid against Stiles's sweat-slicked back.
Derek's teeth close over the edge of Stiles's ear as he shudders and presses tight against Stiles's ass, as deep as he can get. Stiles's dick throbs almost painfully as the conflicting feelings of full and tight and too much wrack his body, but it's done and Derek's hand is locked onto Stiles's hip like he couldn't move even if Stiles begged for it. Stiles has no choice but to ride through the escalating pressure and wait for Derek's tense shivers to subside.
Derek rewards his patience by pulling him back onto his lap, curling one arm securely around Stiles's waist and running another hand gently through the small thick of hair on Stiles's chest. Derek's breath is heavy against his neck, the sounds of his exertion music in Stiles's ears.
Stiles relaxes into Derek's embrace, still in awe that he can get this, that he feels like it's Derek's way of silently praising him for taking his first knot so well.
Eventually Derek's warm breath on his neck moves into soft kisses at the most sensitive stretch of skin and the hand on his chest trails down to stroke Stiles's length. Stiles shivers in Derek's tight grip, very aware that he's still speared on Derek's cock, can still feel it pulsing at his rim.
Derek doesn't make it easy on him, stroking him slowly, unhurriedly, until Stiles is close to crying for release. Then he tips Stiles over the brink. He licks the come off his hand, his cheek pressed to Stiles's jaw, close enough that Stiles can smell himself.
Stiles doesn't tell him it's his first time. He doesn't think he needs to.