There are only two common words in English starting with 'su' pronounced as 'sh': 'sugar' and 'sure'. In 'sugar', the 'su' is pronounced 'shu', and in 'sure', it's pronounced 'shur'. This is a linguistic irregularity in English where 'su' normally makes the 'su' sound (as in 'sun' or 'supper'). All other 'su' words maintain the regular 'su' pronunciation.