Ya... i need to expand on that in the post - we have that sort of understanding of how the averages fail ('average temperature in a hospital'), but we don't seem to apply it well to correlations (which are still just averages).

edit: your brief note is great. You can expand on something popular - e.g. IQ tests - consider different IQ scores and what they actually tell about probability of individual doing this well or this badly on another IQ test (using correlation between 2 IQ tests). Or assuming that there is some 'IQ' that IQ tests correlate with, what does IQ test actually tell about the IQ.