We have already (in Chapter 12) raised the important question: how can we possibly check on our models of the internal structure of the Sun? After all, the interior of the Sun is surely one of the most inaccessible parts of the world we live in. So it is natural to raise the question: how do we know we are on the right track? Could it be that our calculations are far from reality, and are just plain wrong? Is it possible to check on these calculations?