Originally Posted by Meeper

Yea the liquid turned the humans into aliens, which is why they were so secretive of making it (if you cant believe that, but believe that the aliens were real I don't know what to say). They knew the humans wanted to use their weapons and the discovery of that liquid would stop him from being able to go home. As for the fuel they made, it was simply to get the small spaceship off the ground and into the mothership, which is how they managed to finally end up getting back into the mothership.