Mark Twain monostich

~   a pink trip slip   for a three-cent fare  ~

The above is an early documentation of the phenomenon of the earworm, one that drove Twain crazy.  The phrase itself has been rendered innocuous by extraction from its original maddeningly sing-song context -- like a vaccine of capsid protein minus the viral RNA.  Indeed, we can characterize this little wordstring as being deadly as a ditty, but harmless as a monostich.

The connoisseur  will note the rhythmic and assonantial similarity to the classic definition of the made-up word blivvit: 

~   six pounds of sh*t   in a five-pound bag   ~