As a rule, you should try not to end a sentence with a Preposition (in, on, at, by, for, to, with etc.),
 
so do not write or say :
 
“When I leave the office, I turn my computer off”,
 
but rather :
 
“When I leave the office, I turn off my computer”.
 
I recently heard a story in which an intelligent and smart Afro-American from a poor neighbourhood
&nb! sp;
was accepted to the prestigious University of Harvard.
 
On his first day, he approached two more typical Harvard students and asked them, in his rather ghetto-style English :
 
“Say, do you know where the library is at ?”
 
One of them answered in a very patronizing way :
 
“Here at Harvard we do not end our sentences with prepositions.”
 
The new Afro-American student thought for a couple of seconds and then asked again,
 
this time making sure the sentence did not end with a preposition :
 
“Say, do you know where the library i s at, asshole ?”
 
Moral : If you are going to break the rule, at least do it with style :)