Look out, it's more preposition humor. I stole this one from joe-ks.com, via Ben Yagoda's book, If You Catch an Adjective, Kill It:

Sent to prison as a first-time offender, an English student was told by a longtime inmate that if he made amorous advances to the warden's wife, she would get him released quickly. "But I can't do that," he protested. "It's wrong to end a sentence with a proposition."

Okay, let's make it a double; this one is also noted in Yagoda's book. Winston Churchill, having been corrected after violating the sentence-ending preposition rule, supposedly replied with: "That is the sort of nonsense up with which I will not put."