Now look at that figure with the Jason and Topex data.
Cover it all with your hand (don't smudge the screen) except the last 21 little green dots. Make a conclusion about the Earth's long-term climate trend based on just those 21.
Now, remove your hand and look at the 20 previous years.
Do you see a long-term trend? (yes)
Do you see any other sets of 21 points that if looked at all by themselves indicate falling sea level? (yes)
What happens *after* those 21 you just picked? (rises again)

Which tells us more about Earth's climate; any set of 21 consecutive points you might carefully select, or the entire 20 year time series? (I'll let you answer that one)