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Sam is a girl and Dean is a baby and Castiel is taking care of him and trying to protect him while Sam is trying to look for cure or something to fix Dean and himself. Even though this is Castiels little prank he pulled, getting back at both them for last year's April fool's.
His prank lasts a little to long, let's say about..... two weeks, because Dean was just so chubby and adorable and Sam looked feisty and could get any guy (except of course Castiel because he is gay for Dean) to sleep with him.
Of course Sam changed and fed Dean when ever he was with him, which was a lot because he was looking for something to fix both Dean and him, and the fact Castiel was away a lot doing God knows what
(laughing because Sam looked like a depressed mom that has had to much on her hands lately with her knew kid and husband problems staying up all night)
Until finally Sam notices something and finds out getting pissed at Cass because Sam was tired from Dean crying and not stopping unless it was Castiel doing the holding and feeding (Castiel refused to change and poofed out leaving Sam to do the dirty work)
Dean had no memory of what happened and only remembers when he fell asleep. Waking up on a different bed all the way in another town somewhere sleeping while Man Sam was sitting at his laptop doing research. Sam had remembered everything that had happened for the two weeks but decided to mention nothing to Dean since that would make things awkward. But deep inside Sam knew he would have to spill the can with the life they lived one day.
Sam had gotten so use to baby Dean he would at times randomly tell Dean (who was going to get pie) to get some formula ( fuck breast feeding) while he was out, thinking he was talking to Cass. Dean of course would give him a confused look but sam would wave it off. Dean was getting suspicious but soon everything passed. That is until Dean found diapers in the trunk of the Impala.