Why do we say razzle-dazzle instead of razzle-dazzle? Why super-duper, helter-skelter, harm-scarum, hocus-pocus, willy-nilly, fully-gully, roly-poly, holy moly, herky-jerky, walkie-talkie, NIMBY-pamby, mumbo jumbo, Luisa-goosey, wing-ding, wham-bam, hobnob, razzmatazz, and rub-a-dub-dub? I thought you'd never ask. Consonants differ in "obstruct"—the degree to which they impede the flow of air, ranging from merely making it resonate, to forcing it noisily past an obstruction, to stopping it up altogether. The word beginning with the less obstruct consonant always comes before the word beginning with the more obstruct consonant. Why ask why?

The Language Instinct: How the Mind Creates Language