Pedestrian: Wheres the other wheel?
Me: Up yer ass!
“Where’s your other wheel?”
I pull out a little model airplane wheel from my pocket and say:
“It’s right here, you can have it for .75 cents or two for a dollar, but you’d better hurry 'cause they’re going fast!” 
I just smile and don’t be a jerk.
But that’s just me.
What do you consider being a “jerk?” I think giving a witty comeback shows a good sense of humor, and that you can think on your feet…er…wheel! Maybe if you got upset and started an altercation then yeah, that would be my idea of being a jerk. But just engaging in harmless patter and giving the person a laugh, that’s something this world needs more of, not less! And so far I’ve gotten mostly smiles and laughs from people who have asked “where’s your other wheel”, or similar stuff, when I answered them back with something witty,or equally silly.