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A cocky way of saying you're about to prove the other person is a brainless donkey who should be thrown off a cliff for not knowing something basic.
I think you'll find that the Earth is round, not a flat pizza.
I think you'll find that water doesn't flow uphill unless you're drunk.
I think you'll find that 2 + 2 is 4, not 5 because you're too lazy to count.