A/N: Yeah, so I had this idea and I couldn't let it go so I decided to just write it down. Maybe I'll write a third situation too, someday. That is if I can find the inspiration. Anyway, enjoy, leave a review, favorite, whatever. I'll love you for it!
This story is set somewhere in the beginning of season 9. Cas is in the bunker with the Winchesters and still has his trench coat (it's essential for this story! No, not really. Ok, maybe a bit)

Warning! English is not my native tongue; beware of the mistakes, blablabla, and no beta.

1.

Technically, it was all Sam's fault.
Sam had been complaining about lack of good music (seriously, what was he even saying) and had insisted on turning on the radio every now and then, so they could listen to music he liked. Dean had (grudgingly and while complaining loudly) eventually agreed under Cas' stern glare, who apparently liked Dean's taste in music just as much as Sam did. That's why Dean was now listening time to 'Hot In Herre' for the third time that morning. Couldn't they even try to get a little variation? He sighed heavily and got up from his seat. "I'm taking a shower" he said when Sam looked up questioningly from his work. Sam shrugged and continued whatever boring research he was doing and Dean almost ran his way to the bathroom, eager to get away from the horrid sounds the radio was making.
But when the hot water was pouring over him, he had to admit that the song got stuck in his head. And honestly, when he said 'song', he just meant the one sentence he could remember.

Castiel had tried working in the same room with the brothers before, but quickly found that their constant bickering annoyed him well enough to prefer working alone in his room. After Cas left, the brothers had another brief argument about the radio, because, according to Dean, if Cas was gone, Sam had no right to choose the music anymore. The argument ended with a rock-paper-scissors game which Dean inevitably lost.

Thus it was that morning Cas was scribbling, alone in his room, the translation from a Latin text in his notebook when he winced at the cramp in his hand he got from all the writing. He dropped his pen and flexed his ink-spotted fingers. Deciding he might as well take a break to wash his hands and get something to eat, he got up and headed towards the nearest sink, which happened to be in the bathroom Dean was showering in. Cas heard the shower and decided that maybe it was best to knock first. He heard a shout and assumed that it was Dean telling him to come in. The room was hot and damp, and Cas almost regretted wearing his trench coat, when suddenly-

"IT'S GETTING HOT IN HERE, SO TAKE OFF ALL YOUR CLOTHES"

Cas jumped up in surprise and stared at the shower. Take off his clothes? He didn't know people did that when they were in the proximity of another human being. Dean must feel very comfortable around him. His tone sounded a bit off, though. He hesitated and opened his mouth to protest when-

"IT'S GETTING HOT IN HERE, SO TAKE OFF ALL YOUR CLOTHES"

Well, It was indeed getting really hot inside the small bathroom. So Castiel took Dean's advice and started stripping. First he shrugged off his trench coat, then he loosened his tie and unbuttoned his shirt. That felt much better indeed. He was just about to wash his hands when again he heard:

"IT'S GETTING HOT IN HERE, SO TAKE OFF ALL YOUR CLOTHES!"

ALL of his clothes? Surely that wasn't necessary? But apparently Dean insisted, so Cas threw off his shirt, unzipped his pants and stepped out of them, contemplating on whether he should take off his underwear too or not. He decided it was best to just ask Dean.

"Dean, should I-" But he was interrupted by a sharp yelp and suddenly Dean's head appeared from behind the shower curtain.

"Cas, what the hell? What are you doing here?" His eyes were wide as he saw Cas and looked at him from head to toe. Before Cas could reply Dean added "And why are you almost naked?"
Now Cas was really confused. He cocked his head with furrowed brows. "Because you asked me to." Dean looked at him in confusion but then realization dawned on him. "I was singing, Cas!"
Oh, so that was why he sounded so off. It didn't really sound like singing either, though. "You were singing?" Dean blushed ferociously.
"Yeah well, we can't all have the voice of an angel" But as far as Cas knew, he didn't really have a nice voice. He opened his mouth to protest but Dean blushed (if possible) even more and quickly added: "I mean we can't all have a nice singing voice." Cas nodded in apprehension.
"But why didn't you knock?"
"I did knock, Dean, and you shouted an answer. I thought you said I could come in."
"I was singing!"
"My apologies, I wasn't aware of that"
"Yeah, well,…" he trailed off and gave Cas's body a lingering look. But then he realized what exactly he was doing and that he himself was actually still naked (but luckily his body was covered by the shower curtain) and snapped his head back up. "Just get out of here, will ya?" So Cas picked up his clothes and fled the bathroom.

Dean made sure to ask Sam if Cas could borrow his IPod for a while, so maybe he could understand when he was hearing lyrics instead of orders.

2.

Since Cas wasn't familiar with human food, and didn't really know what he liked (except for burgers, but although Dean wouldn't mind eating burgers every day, Sam insisted on varying in their food), Sam and Dean took him on a trip to the grocery store and told him to pick whatever looked tasty to him. That didn't really help. The only thing Cas was persistent in buying was a box of Honey Nut Cheerios. He insisted it was because it was the healthiest-looking cereal in the whole rack, but Dean suspected it had more to do with the happy bee mascot adorning the front of the box. Apparently Cas' obsession with bees hadn't completely vanished along with his derangement.
He really did seem to like the cheerios, though. Soon he didn't only eat them at breakfast, but also as a substitute for popcorn when they were watching tv. And watching tv seemed to be rapidly becoming one of Cas' new obsessions.
Anyway, since those cheerios were his new favorite all-time snack, Cas brought them with him everywhere. He always had one –if not two- of his trench coat pockets stuffed with cheerios and would occasionally pop one into his mouth. Now that wasn't too bad.
But with this snack came a bit of a disadvantage: Cas left a trail of cheerios everywhere. That's pretty understandable, Dean thought. After all, as an angel he probably never really had to clean up anything, save for blood stains maybe. And maybe being human meant having some bad habits for Cas. Maybe the human Cas was a bit messy. So he send Sam to explain Cas what cleaning was for, and apparently his speech worked, because Cas started paying attentions as to where he was dropping his cheerios, and when he did, he picked them up and popped them into his mouth. Dean tried to explain that eating from the ground probably wasn't such a good idea, but at Cas furrowed brows and gave him his puppy-eyed look so he just shrugged and let him have is weird (and maybe a bit gross) habit. They did manage to convince him not to do it when they were outside, though.
The bunker stayed clean for a little while, but after a few days Dean and Sam started finding cheerios again. This time at random and weird places. The first time they found a cheerio on the television. They looked at each other, raising their eyebrows, but then they just shrugged and decided to just ignore it. The second time, Sam found a cheerio on his laptop, and when he opened it, the screen was frozen on a site called , more specifically a story about Dean and Castiel. Curious, Sam started to read, but quickly he reddened, then became white, and suddenly he got an unhealthy looking green color on his face. He closed the tab, remembered to delete his history and went to the bathroom to brush his teeth (because he threw up in his mouth a little) and maybe even wash his eyes. He considered all the jokes he could make based on what he just found out, but decided against it. If Dean read what he just read, he could be traumatized for life (and maybe Sam would have been if he hadn't stopped at the point things were really starting to get dirty).
The third time, Dean was just stepped into the shower when his foot crushed something. He looked down, grabbed his foot and sure enough, there were bits of cheerio sticking at the ball of his foot. The shower? Really? When he saw Cas a little while later he shot him a weird look, but said nothing of it. The guy was doing his best, what more could he expect him to do?
But when he woke up one day after he fell asleep on the couch and wiped away the dried remains of his drool (which weren't there. Obviously. Dean didn't drool) ,found himself covered in cheerios and Cas hovering over him, he decided this just couldn't be some random sloppiness. No. In fact, the cheerios were carefully placed upon his chest , the little loops forming one big circle. Dean shot up and the cheerios dropped to the ground.
"What the hell, Cas? What is going on?"
On the other side of the room, Sam tried very hard to stifle a laugh.
" Apparently, if I like it, I should put a ring on it"
Dean's mouth fell open and Sam sprayed coffee all over the table. "I'll just leave you two lovebirds alone then", he said and disappeared. Dean was too much in shock to comment on the mocking words. When Dean remained silent, mouth still hanging open and eyes wide, Cas started shifting uncomfortably.
"I just thought that with you, because I really like you very much, the combined rings forming one big ring would have the desired effect." Dean shook his head and grabbed Cas's shoulder. "Don't take those songs too literally, Cas" And then he hurried out of the room.

When he found Sam, he rolled his eyes. "Dude. Beyoncé? Really? That's what you're letting him listen to?"

THE END (for now?)