Everything was going well; Cas was on top of Dean, holding his legs up and away from where he was thrusting. Dean was moaning, his head tossed back, and Cas was biting his lip to keep from making noise himself.
Then it hit him; he remembered a post on Tumblr, which he'd recently discovered, called 'things to say during sex'. They didn't make much sense to him, but he was new to Earth, and perhaps Dean would understand them.
He bent down seductively, slowing his thrusts, and murmured the words in Dean's ear.
"Do you feel it now, Mr. Crabs?"
There was a pause after he said it, where he waited for Dean to respond, then laughter burst forth into the room.
Dean immediately went soft, and pulled away from Cas, literally rolling side to side on the bed. He was roaring with laughter, and Cas couldn't understand why. The Tumblr had told him to say that, why would it give him bad advice?
Confused and slightly hurt, he watched Dean until his laughing died down. It took far too long; there were a few times when he quieted, but then he looked at Cas, repeated the phrase, and cracked up again.
Finally, when he was irritated enough, Cas spoke up.
"What is so funny?"
Dean giggled another few minutes before explaining to him why he was laughing so hard. By the end of the explanation, Cas chuckled too.
"So what did you learn, Cas?" Dean teased him.
"Never trust the Tumblr."
"That's right."