For some obscure reason English speakers will almost always find a flip-flop to be more natural than a flop-flip, or a pitter patter of tiny feet to a patter pitter, or a tick tock to a tock tick. When the vowel changes in a reduplicative term — such as wishy washy or hip hop — it's known as ablaut reduplication and the vowels almost always follow the order I-A-O. If you say them in any other way they almost always sound weird. It's quite fun to think of examples.
I learned this neat thing from Mark Forsyth when learning about the even more surprising English grammar convention about ordering adjectives — he was explaining why we say the Big Bad Wolf (thanks ablaut reduplication) and not the Bad Big Wolf as our other grammar convention would dictate.
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