From Zhu Xi's (
朱熹) Southern Song poem
《观书有感》 (On Reading Books). Couplet:
问渠哪得清如许?
为有源头活水来 - 'Ask the canal how it can be so clear? - because fresh water keeps flowing in from the source.' Written after a morning of reading: the clear pool is a metaphor for a clear mind, constantly refreshed by new learning.
渠 here is a classical third-person pronoun as well as 'channel' - poets exploited the pun.