Grocery store checkout line gold
Kid asks me why the checkout person is scanning everything. I said "It's so they know how much it costs." He goes "Oh, so if we don't scan it, it's free?" Had to explain how that's not how shoplifting works to my son in the middle of the grocery store. Lady behind us was dying laughing.
That's a solid setup for the punchline. Did your kid laugh or just stare at you like you were losing it? Those are the best moments either way.