The word "rapozo" is spelled with a "z" instead of an "s" because the sound made by the letter "s" in this word is actually a voiced alveolar sibilant. This means that the vocal cords vibrate while saying the sound, giving it a slightly buzzing quality. In IPA phonetic transcription, the word is spelled /ɹəˈpoʊzoʊ/, which includes the symbol for the voiced "z" sound. This may seem like a small difference, but it is important to accurately represent the sounds of words in written language.