My brother showed me this card trick...?

Question

a couple of years ago.

i couldn't see what the cards were,

but he told me, "put the cards in 2 piles, the red pile and the black pile, without looking at them."

so i did.

two piles of cards that i couldn't see.

he flipped them over,

and i was mostly correct, except there were like, 2 red cards in the black pile & vice versa.

would anybody care to explain to me how he did this?!

Answer 1

answer

Answer 2

The cards were already arranged so that the cards went red, black, red, black, red, black etc.