Two barmen in matching polo shirts, one small, one tall, stand behind the bar with arms folded engaged in debate with a regular sat at the bar.
The tall barman leads: “No, you’re not getting what I’m saying: I’m asking, does a staircase go up or come down? Which way does it go?”
“Up,” says the baffled regular. “If it didn’t go up, you wouldn’t need it to come down. That it comes down is a side effect of it having gone up in the first place.”
“No, it’s both. It goes up and comes down. It doesn’t matter that it was built specifically to go up. Once you’ve got an up, the staircase has to go down as well. So it goes both ways.”
The small barman frowns, laughs quietly, and shakes his head.
“What are you on about? What are you actually on about?”
“Alright, scratch that, here’s another one: is a zebra black with white stripes, or white with black stripes? Eh? Think about it.”
The regular says, confidently: “White with black stripes.”
“Yeah, but how do you know for sure?”
The small barman claps in delight.
“He’s got you there, mate!”
“Alright, what about this one: we’re all agreed stairs go up and down–”
Regular: “No, but carry on.”
“– but what about escalators? Does an escalator go up, or come down?”
“You’ve hoisted yourself by your own petard here,” says the regular. “It depends which way it’s going, doesn’t it? I mean, you literally get one to go up, and another to come down.”
“Ah, see, no, you’re wrong, and I’ll tell you why: because it has to come down on the underside or it can’t go up. It’s a loop. So escalators always go up and down, just like staircases. Makes you think, doesn’t it?”