Ablaut reduplication

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.

A possible explanation is that in making the sounds we start at the front of the mouth and move to the back and that feels slightly easier or more natural as we make the sounds. Try it!