The way I heard it

There’s a small plaque on every dashboard of every spaceship.  They are not easily decipherable of course because they’re written in alien, but what it translates to is:

Please Don’t Litter

They take this very seriously and never in history have we found anything they have left behind.

What did take place some time ago was this alien, we’ll call him Larry, well he wanted to let us know that they had been here and the only thing he could think to do was to pile up some rocks. So he did.

Unfortunately, just like here on Earth, there’s always a jerk, someone who just has to mess things up.  Outer-space is no exception.  Scooter, having seen what Larry did cast an alien spell on an animal that happen to be walking by.  This will show these Earthlings who’s boss, he thought to himself.

We know through scientific research and stuff that the animal that happened by on that day was the common house cat, and we further know that the evil alien spell prevents this species from throwing up on tile floors.  It always has to be on carpet.

So now you know.