They call that a paradox! the departed quote?

Costello asks young Collin "Do you go to school?" he says yes and Costello replies "That's good. They Call that a paradox". how is it a paradox? i don't see how the 2 things contradict with each other. i know there is know way they could make a mistake like this nor would Jack Nicholson... show more Costello asks young Collin "Do you go to school?" he says yes and Costello replies "That's good. They Call that a paradox". how is it a paradox? i don't see how the 2 things contradict with each other. i know there is know way they could make a mistake like this nor would Jack Nicholson say a quote like this if it doesn't make sense - as far as i know he's quite erudite.
if someone could explain this to me, like one would to a 9 year old i'd appreciate it.
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