That change, called an oscillation, can happen only if neutrinos have a tiny amount of mass. Neutrinos, because there are so many of them, thus become the prime candidate for the missing matter of the universe. Astronomers haven't to this point discovered enough matter to keep the galaxies, collections of 300 billion stars, from flying apart. Since galaxies like our Milky Way are NOT flying apart, they really need to find that matter. Our knowledge of what will happen to the universe depends on a tub of water in a mine shaft in Japan.
Most people take for granted that the sun and all the stars are hydrogen bombs.
But until physicists can detect the right number of neutrinos coming from the sun, we can't be absolutely certain that the hydrogen-bomb model is correct in all its details. Sure, the study of neutrinos won't produce Velcro or Viagra, as last week's AP story gleefully points out. It's just the key to our understanding of the universe. It's OUR universe, folks, the only one we have. How it formed and how it works ought to be of passing interest to us as we watch our Seinfeld reruns, take our Viagra, and slather Rogaine on our heads.