(Thanks to Lulu for this one)I don't know if it's true, but I've heard that:

Shànghǎi zìláishuǐ láizì hǎi shàng

上海自来 水 来自海上

(Shanghai's tap water comes from on top of the sea)

I'm all in favor of fun little word games and/or tongue twisters as a way of learning new vocabulary (tap water), practicing pronunciation, and earning the undying respect and admiration of friends. Well, at least I know it works for vocabulary and pronunciation.