Opening to an explanation, typically a contradiction, where the speaker is implying in a none-too-subtle manner that the other person could well be among the stupidest person in existence (ever) for not knowing this.
Dude A: "I'm pretty sure that 2 and 2 is 4"
Dude B: "I think you'll find it's at least 5."
Subtext: "I am superior in every way. You have shit for brains