First off, we start with a Ruby.
It's the start of all good stories, really. Imagine it beautiful and regally suspended, and wrap it in material that reflects perfectly the light, and give it a shape that refracts the light equally as perfectly.
And then imagine sending a searing beam of a very pure light inside the encasement wherein the ruby sits suspended and perfectly cut.
The ruby goes nuts, of course.
You can imagine.
All this light that never leaves reflecting on and in and through itself over and over without end.
The Ruby begins to change the light, to accept it: in particular, molecules inside the crystal matrix become excited and begin to change the way the light moves. They seem to store the light momentarily, and then they emanate it, a light changed, through a tiny opening in the reflective encasement.
When that light leaves, that light folded and reflected upon itself many billions of times and mixed with the vibrating, it becomes pure and of a singular nature.
And they call that singularity a Laser.