It all starts with a question.
And, okay, they can't really blame it on him. Castiel is turning human, which is scary and infuriating and he'd give anything to stop it from happening; but it is happening, things are changing inside him, so he's curious. Curious about eating, and sleeping, and money, and cars, and computers, and things he's never needed to think about before. So he asks questions, and because he's mostly only around Sam and Dean, Sam and Dean are the ones who have to answer. Fair enough. They're his friends, after all.
But even though all his questions are perfectly innocent in intent, Cas seems to have developed a remarkable knack for asking about things Dean really, really would rather not discuss at all, especially with a socially retarded almost-ex-angel. Things like "what are feminine hygiene products? Why are there not masculine hygiene products also?" which leads to a super-fun lesson about the female reproductive system in the middle of the grocery store, since Cas clearly hadn't paid any attention during angel sex-ed; and "what does slut walk mean?" which unexpectedly reveals Sam's inner hardcore feminist; and, so far Dean's personal favourite, "what's a Nikki Minaj?" though he's a little less pleased by the fact that Sam insists on showing their friend every video of hers he can find.
He asks other stuff, of course, non-awkward things like what do you do with shampoo and how do you put gas in the Impala, so when Dean realizes his curiosity has been piqued yet again he's not immediately suspicious. They're driving back to their current motel after successfully taking out a nest of vamps, and since he's basking in the glow of a hunt successfully completed as well as focusing on the road, Dean hasn't really noticed the way Cas cranes his neck to stare at a plaza as they pass it by.
Just a regular plaza, a handful of fast-food places with one real-ish restaurant, a suspicious second-hand bookstore that Sam eyes longingly, a computer store, an athletic store, a few other places. But when Cas finally tears his gaze away he's frowning pensively, clearly deep in thought about something or other.
"What's up, Cas?" Dean says cheerfully. "You want a pair of yoga pants or something?"
"I don't know what that is, but probably not," says Cas, since as usual the joke has soared gracefully right over his head. "I was just wondering…I thought toys were designed for children."
Dean and Sam exchange glances, which is enough to solidify the knowledge that none has any idea what Cas might be talking about. "Uh, they are," Dean tells him. He doesn't mind volunteering himself to field what seems like a safe enough question; at least if he takes a few then he can feel a little less guilty about leaving Sam to deal with the uncomfortable body-slash-sex questions.
"And the sacrifice of one's toys is a mark of passage to adulthood, correct?"
"I…guess so…" Dean thinks of Sam, whose "playtime" growing up tended to consist of learning how to shoot straight and packing cartridges full of rock salt. Those certainly aren't toys he's given up—though in the Winchesters' case it was more a matter of childhood getting roughly shoved out the door. They never had to worry about how to grow up.
"I see," Cas says in a way that means he doesn't see at all.
Dean sighs, and since they seem to be dancing around something here he just asks, "Why? What's up?"
"I just don't understand," Cas admits after a moment's internal struggle as he tries to sort human things out on his own, "why anyone would design an adult toy store."
Oh. Okay. So obviously one of the stores they just passed was a stag shop, and now someone's going to have to explain to Cas what a dildo is. Fantastic.
Luckily this is where Sam steps in. It's weird how this whole thing works: Dean has more sex, has kinkier sex (he's pretty sure about this, anyways, though he's never actually had an opportunity to find out and really doesn't ever want to) than his little brother—yet something about discussing it with Cas makes him squirm. Sam, on the other hand, is not only remarkably knowledgeable but also in possession of an attitude towards sex similar to that of one of the better kinds of public health nurses. He's like the cool older brother Dean never had, the one who can answer all the shy questions and give the kind of advice that you actually need but never seems to get mentioned anywhere else. Only Cas isn't shy because he doesn't know he's supposed to be, and he tends to ask more out of interest than out of a genuine need for practical information; but Dean still can't help getting uncomfortable when Sam is graphically explaining to Cas how different types of birth control work or what "getting head" means, as well as feeling weirdly left out. Maybe because he's used to Cas coming to him for help, not his brother; maybe he just wishes he were as open as Sam. Anyways, whatever. The whole thing is just frigging weird.
"It seems counter-intuitive," Cas continues, utterly oblivious. "And also unprofitable."
So obviously Sam steps in here, twisting around in his seat to tell Cas, "They're different kinds of toys. Things to help with sex."
"I wasn't aware humans required any assistance in that area."
"Well… not require, no. Just to make sex more fun, I guess. To make it different. Or for people who're single."
Which leads into Dean gritting his teeth and watching his knuckles go white from gripping the steering wheel so hard, while Sam explains in great detail to Cas how vibrators, and cock rings, and butt plugs, and anal beads, and a ton of other things Dean doesn't even want to think about work, and Cas just sits there with his head tilted to one side asking questions every once in a while for clarification, so by the time they finally, finally get back to the motel Dean practically leaps out of the car, calling back that he's going for a walk and not to wait around for him.
By the time he gets back the sun is going down, and since none of them ate dinner they're all hungry. Sam has stopped talking about masturbation and anal sex and similarly fun things, for which Dean says a silent prayer of thanks. Cas may still be considering his latest fun-things-you-can-do-with-your-dick lesson from Sam, but for the moment at least has been mercifully distracted by a sudden urgent need for burgers. Dean breathes a sigh of relief and decides it's probably safe to go for dinner with them, even if he has to spend the whole meal pointedly not imagining what Cas would do with a vibrator.
They're halfway through their meal when a disgruntled Cas leaves to go to the bathroom—probably his least favourite aspect of being human, maybe because it's so damned impossible to ignore. Dean's happy to concentrate on his burger, which isn't especially remarkable but hey, it's a burger and burgers are amazing enough that he's been trying to work out how such a shitty world allows for their supernal existence for years by eating as many as possible. Sam, on the other hand, waits till Castiel is out of hearing range to set down his fork in a we-need-to-discuss-something way, and then asks, "How come you never want to talk about any of this stuff?"
"What? Are you… I love talking about food. If you haven't noticed over the past, like, twenty-six years of me being your brother."
"Not about this." Sam waves a hand dismissively at the plates in front of them, which in Dean's opinion is verging on sacrilege. "About other stuff. About all the stuff Cas wants to know."
"Okay, first of all, not true. I definitely taught him how to text yesterday." For two frigging hours—honestly, you'd think someone as powerful and ancient and kind-of omnipotent as an angel of the Lord would be able to figure out a simple cell phone. Apparently, however, his technological ability falls somewhere below that of an eight-year-old child. By now, in fact, Dean has almost concluded that Castiel's learning curve for computers is basically just a straight line. "And second of all, why the hell would I want to talk about…stuff like that, like this afternoon…with Cas? Jesus. You're the one who's all weird and teacher-y with it."
Sam looks at him for a long time. Dean hates this, the way Sam can see right through him to things Dean didn't even know were there; he's trying so, so hard not to fidget, even though that's what he does when he's uncomfortable and Sam staring at him like this is definitely making him uncomfortable, because he knows Sam will just read into that, as well. "I'm not being weird and teacher-y," says Sam eventually. "I'm just telling him what he needs to know. I'm just being for him what you were for me."
And that shuts Dean up for a minute. Sam's like the cool older brother Dean never had? No, Sam's like the cool older brother Sam had, the one he looked up to, the one who showed him the ropes since their father was too busy. Dean remembers teenage Sam coming to him to ask self-consciously about what his body was doing, and about girls, and about sex; and since he was blushing the whole time, clearly mortified like most kids his age, Dean had to be cool about it. Laugh off his embarrassment—show Sam there wasn't any reason to be embarrassed in the first place, since it happened to everyone sooner or later.
"It's natural stuff," Sam adds. "That's what you used to tell me, anyways. And, like." There's another pause as he struggles to come up with a way to say what he's thinking. "You just…I don't get it. How come you could talk about this so easy when you were seventeen, and now it's like you've turned into some first-grader who still thinks kissing is gross?"
"I…what? Kissing's not gross," Dean protests feebly. It's all he can think to say. The conversation seems to be veering dangerously close to those things other people talk about but that he definitely doesn't have. Emotions or whatever. Ugh.
"Well, yeah, I know. And neither is sitting down and explaining what a boner is to your kind-of human friend who's freaking out because he thinks his arteries are going to explode."
"Yeah, but, like… it's…you know. Cas."
"Well, exactly. You two are supposed to be friends. It's not like Zachariah's coming to ask you for dating tips or something. I mean, Cas is family, right? So I was expecting you to just go into experienced big brother mode automatically."
Okay, it's a little bit of a valid point. What makes this so different from helping out his younger brother? Or, for that matter, from taking Cas to a brothel not so long ago in an attempt to help him get laid? He might say he's just out of practice—Sam hardly needs his help in those matters anymore, and all he really had to do in the brothel was give Cas a shove in the right direction (though look how well that turned out); only the other day when Sam was out getting groceries Cas had woken up pretty damn rock hard, and after being asked rather mournfully how he was supposed to get rid of it Dean had blushed and stammered and generally made a complete idiot of himself as he attempted to explain the concept of masturbation before fleeing the room as soon as Sam returned. There's just something about putting Cas and sex together that makes him go hot and cold and his hands start sweating and it's hard to focus properly. Or maybe not Cas and sex, exactly, since he was fine with trying to set Cas up with Chastity that one time; maybe more like Cas and Cas's body. Cas's body that he's been unintentionally seeing a lot more of recently. Cas's body that sometimes wears his old clothes since there are only so many days you can go in a suit and trench-coat, and Dean kind of likes the way it looks on him. Cas's body that he has to try not to imagine naked in the shower every night, covered in soap and hot water. Cas touching his body—
Fuck it. Cas's body that really, seriously, excruciatingly turns Dean on sometimes.
But, like, whatever, right? Cas is hot. Fine. Dean can deal with it. He's not some sissy teenager moping around because he's in love with his best friend or anything. Chris Evans is hot too, and Dean managed not to curl up in a sobbing ball after he saw Captain America just because he'd never get to have sex with him. He's perfectly content to keep it his not-so-little secret, to admire from afar (or at least as afar as the personal-spatially-challenged angel will allow)—except it's kind of hard (pun definitely not intended) when Cas is asking in innocent bewilderment why on earth a man would possibly want a vibrator and Sam is answering unashamedly, as if his comfortably straight younger brother has ever wanted to try so how does he even know this stuff, and Dean is listening to everything Sam says even though he doesn't want to and even though he really doesn't want to he's also picturing Cas doing everything Sam is explaining in careful detail.
So yeah, he prefers to leave the sex talks to Sam.
Cas reappears, wiping his still-damp hands absently on his (Dean's, actually, which isn't helping) jeans, and all Dean has time to say before he sits down with them again is a vague, "You're way better at it, anyways."
"Better at what?" Cas asks, who is forced to dip a few of his fries in ketchup now that he's demolished the entire burger.
"Nothing," Sam says quickly. It's both a blessing and a curse—they won't be discussing Dean's supposed sudden-onset bashfulness (yeah, right) with Cas right then and there, but Sam will undoubtedly want to continue the conversation later. Fantastic.
"Oh." Ordinarily Cas might have pushed a little further since it's obvious they're hiding something, but right now he's too busy eyeing his empty plate despondently to pay much attention. "Can I have another burger?"
"No," Dean and Sam say, practically in unison.