A former student of mine now living in and working in Russia, Troy McGrath, recently wrote to me and passed on this anecdote:
A linguistics professor was lecturing to his class one day and said, “In English, a double negative forms a positive. But in some languages, such as Russian, a double negative is still a negative. However,” he pointed out, “in no language in the world can a double positive form a negative.” But then a voice from the back of the room piped up, “Yeah, right.”
I responded that intonation was a crucial factor in his example and gave him a second example I actually heard.
Another linguistics professor, the late Kenneth Pike, once proved the importance of intonation in speech by demonstrating that intonation may contradict the content of a sentence.
If we simply say, “I love you”, the sentence has a positive meaning. But if we add question intonation, “I? Love you?”, the meaning of the sentence is exactly the opposite of the content of the sentence.