If a little kid ever asks you just why the sky is blue, you look him or her right in the eye and say, "It's because of quantum effects involving Rayleigh scattering combined with a lack of violet photon receptors in our retinae." --
They say that even the brightest star won't shine forever. But in fact, the brightest star would live the shortest amount of time. Feel free to extract whatever life lesson you want from that.
I am using the word as a scientist means it: a set of ideas so well established by observations and physical models that it is essentially indistinguishable from fact. That is different from the colloquial use that means "guess." To a scientist, you can bet your life on a theory. Remember, gravity is "just a theory" too."