This is a scatter plot showing the amount of sleep needed per day by age.
As you can see, as you grow older, you need less sleep (but still probably more than you currently are getting...).
What type of correlation is shown here?
Estimate a line of best fit.
Using this line, predict how many hours of sleep a 22 year old needs.
Using this line a 22 year old needs about 7.5 hours of sleep.
Using this line, predict how many hours of sleep a 6 month old child needs.
Using this line a 6 month old (½ year old) needs about 14.5 hours of sleep a day. This is probably less than needed, but it's generally in the ballpark.
Can we use this line to predict how much sleep a 40 year old needs?
Since this data is not quite linear (i.e., falling in a straight line) if we use it to predict ages outside the range provided, it will probably be inaccurate. If we extend both axes, here is what this line would predict for a 40 year old:
Obviously, a 40 year old needs more that 2 hours of sleep/day. This is an example of when we cannot use this graph to make a prediction.