We have quite a serious problem with students taking stuff off the internet here.
Is here the UK?
(I haven't seen any Cliff Notes in a while, but I don't imagine they have changed much. If, for example, you haven't read Romeo and Juliet the Cliff Notes on the play would explain the play and explain the motivations of the characters, etc. Presumably, you would know those things if you had read the play and paid attention in class. However, with the aid of Cliff Notes you have to do neither. (True, some people can read the play and still not pay attention; thus they can read "Romeo, wherefore art thou" and think Juliet is looking for Romeo even tho the context gives no indication that is true.)
Here is the UK. I once had a piece handed in by a student that had been copied wholesale off the internet. I found the site, printed it off and handed it back to the student with the URL at the bottom.