Shu is composed of 孰 shu, as phonetic, with 火 huo fire underneath as radical. This phonetic shu was the original character for cooked, ripe; but inasmuch as it was also used for another sound shu meaning who? what? the two senses were separated as time went on by the insertion of the radical fire whenever shu meant cooked or ripe, to distinguish it from shu who? what? Thus it was that the growing exigencies of the language called into existence new characters to divide the burden of meanings.