This document discusses phonemes and allophones in linguistics. Phonemes are the sounds that distinguish meaning in a language, like the difference between 'sheep' and 'ship'. Allophones are variations of the same phoneme that do not change the meaning, such as the 't' sound in 'sent' and 'cent'. An example is provided analyzing the phonemes and allophones in the tongue twister "She sells sea shells on the sea shore".