Osaka. A man tells you, with total sincerity, that he has one hundred million yen in his wallet.
He pauses. He is looking at you. The whole table is looking at you.
You say "…oh, really?" and the temperature drops eight degrees. You had one job. The correct answer was なんでやねん, delivered fast, with a light backhand tap to his arm.
Japanese comedy is built on a two-person structure: ボケ (boke), the one who says something absurd, and ツッコミ (tsukkomi), the one who calls it out. Manzai duos have run on this for centuries, and in Kansai it leaked out of the theatre and into ordinary life.
なんでやねん is the default tsukkomi. Literally "why is it like that", functionally "mate, what". It comes with choreography: a quick hand movement toward the other person's chest or shoulder, fast delivery, falling tone.
The important part for visitors is that the boke is an invitation. When an Osaka person says something ridiculous with a straight face, they are not lying to you and they are not testing your Japanese. They have set you up, deliberately, and they are waiting for the return shot. Doing nothing is the social equivalent of leaving someone's high five hanging.
Get it right once and something genuinely changes in the room. You are no longer a tourist being accommodated. You are playing.
Everyone learns that Japanese people avoid confrontation. Then you get to Osaka, where interrupting someone loudly is a form of affection and going along politely is a mild insult. It is the same country. The difference is that Tokyo's default is "do not impose" and Osaka's is "do not leave me hanging" — and once you see that, half of Japan's regional jokes about itself suddenly make sense.