They came to earth via spaceships filled with rocks that glowed in the dark. There was a hole on the top and bottom of the spaceship. If the vacuum of space began to suck stuff out of one hole they would plug it and open the other. Then if that one fell prey to the vacuum they would plug it and open the first.
And that's why people should keep both ears plugged when around Mormons or else your brains will get sucked out your ears!