I have been doing some research on text analysis, in particular on extracting sentiment in the absence of context (a la Twitter). Among all the theory, I came across the following joke which I think really captures the complexity of any natural language processing (not that it prevents us from trying):
A linguistics professor was lecturing her class one day. "In English," she said, "A double negative forms a positive. In some languages, though, such as Russian, a double negative is still a negative. However, there is no language wherein a double positive can form a negative."
A voice from the back of the room piped up, "Yeah, right."