孰 (shú) is a classical Chinese interrogative pronoun meaning 'who', 'which', or 'what'. It is rarely used in modern spoken Chinese except in set phrases or literary references. You will encounter it in idioms like 孰是孰非 and in classical texts. For 'who' in modern conversation, use 谁 (shuí); for 'which', use 哪个 (nǎ ge); for 'what', use 什么 (shénme).