No Ending Sentences With a Preposition

There's a joke goes that:
Two ladies, a Yankee and a Southern Belle, are sitting next to each other on a plane. The Southern Belle turns to the Yankee and asks, "So, where y'all from?" The Yankee replies, "I am from a place where we do not end our sentences with a preposition." Without missing a beat, the Southern Belle bats her lashes and asks, "So, where y'all from, bitch?"

A similar one:
A man visiting Harvard: "Can you tell me where the bathroom's at?" Harvard student: "I'm sorry, at Harvard we do not end sentences with prepositions." Man: "Oh, I'm sorry. Can you tell me where the bathroom's at, asshole?"

Here is a fun way to reply when ridicule people who violate this grammar rule: "Right before the 'at'!"

所以，「洗手間“在”哪裡？」英文到底要怎麼說呢？

英文文法第一法則：介係詞不可以放在句子的後頭！

看！連外國人也有這個困擾：
http://grammar.quickanddirtytips.com/ending-prepositions.aspx

http://grammar.ccc.commnet.edu/grammar/prepositions.htm#ending

http://hk.myblog.yahoo.com/siu82english/article?mid=65