If you present liberals with a fact which doesn't fit into their pristinely politically correct schematic, they sometimes respond, "Uh, what's your source on that?"
I don't footnote everything I read or hear, so I usually respond, "I don't remember exactly, but I did read it somewhere."
It's always painfully obvious that they're hoping I'll cite Fox News or some other conservative outlet, so that they can snort dismissively, indicating that anything that source would say is automatically suspect.
So I'm often tempted to respond, "Oh, I read it in Nazi Weekly. Haven't you gotten your latest issue?"
But I usually just take satisfaction from knowing that their question translates directly as, "I'm stymied by your facts, which have proven me wrong, so I'm resorting to this incredibly lame fallback instead."
I sometimes argue with conservatives as well. But none of them have ever asked me for my source.