One night, when Sam is away, Dean is putting in a half-assed effort to get wasted when Castiel shows up. He's just tipsy enough to decide that asking Castiel about unicorns is a good idea.
"So, what's the deal with unicorns?"
Castiel turns from studying, well, Dean's not sure. The telephone or something equally random with an intensity that must be exhausting to maintain. He makes the expression that Dean has dubbed, "intensely serious expression #3". "They became extinct in their great and terrible war with the dragons."
Dean blinks, then snorts. "You're joking."
"Yes." Castiel's expression doesn't change.
The noise Dean makes roughly translates to, "Buh?"
"You have said that you think I do not have a sense of humor. That was a joke I just made up."
"Ooookay," Dean says.
Castiel picks up a knife that Dean had been cleaning and studies it, and Dean decides that it just must be some sort of thing with him. "Actually," Castiel says slowly, "Unicorns and dragons got along well." He frowns suddenly. "Too well. You understand?"
It worries Dean more than a little that he does. "You mean they..." he raises his eyebrows and waves his hand.
"Inter-species relations are," Castiel pauses and frowns more intensely, "discouraged."
Dean is really still stuck on the mental image. "How is that even possible?" When Castiel looks like he's about to explain, Dean stops him. "I don't actually want to know."
Castiel sighs. "God had to smite them all." He shakes his head a little and sets the knife down.
"I'm sorry I asked," Dean mutters into his glass as he takes another sip of his drink. He thinks it's a smile that flashes across Castiel's face, but he can't be sure.