A very long time ago, right here where we are now in the Universe sat another planet just like earth, with people, mountains, rivers and seas.
Though just like earth, it was very much larger. You see, in time,
everything gets smaller. That's why today, we see the stars racing out
away from us, as we slowly shrink down in size. Astronomers call this
Hubble law, named after the very famous Edwin Hubble who lived last
century, who observed this for the first time.
Over the centuries, like ourselves, the people of this planet became
very smart and learned to fly into space high above their planet. But
over billions of years their sun expanded, swallowing their planet, and
they had to leave. For more on this, go here:
http://www.zyx2.org/ALIENS/MILLSTADT.html