Prepositions

A high school senior was inspecting Harvard University, where he hoped to attend the following autumn. 

As he was walking across the Quad, he stopped a distinguished-looking man and asked, "Sir, can you please tell me where your library is at?"

The man looked down his nose and replied, "Son, I'm head of the English department, and I can assure you we don't end our sentences with prepositions. Re-cast your sentence in a proper form and I will reply."

"Can you tell me where your library is at, jerk off?"