Ever wondered why we say tick-tock, not tock-tick or ding-dong, not dong ding?Turns out it is one of the unwritten rules of English that native speakers know without knowing.
The rule,explains a BBC article, is: "if there are three words then the order has to go I, A, O,. If there are two words then the first is I and the second is either A or O. Mish-mash, chit-chat, filly-dally, shilly-shally, tip-top, hip-hop, flip-flop, tic tac, sing song, ding dong, ping pong."
This rule seems inviolable:
" All four of a horse's feet make exactly the same sound. But we always, always say clip-clop, never clop-clip."
This rule even has a technical name, if you care to know it --- the rule of ablaut reduplication.
For more info visit :: BBC.