Relative Reality

Alex and his sister Teena had passed a doorway, and traveled maybe light-years through space. They encountered a flying monster so humongous its weight alone should have made it impossible for it to walk, never mind fly. Yet it floated in the air like it was full of helium.
Then Alex and Teena fell and plunged hundreds of feet into a creek. Now mind you, the water softened the fall, but still! They'd never have survived falling hundreds of feet on earth, not even on the moon with its reduced gravity.
And those aliens! They looked so peculiar. Many of them didn't even belong to the same species. Yet they were all encased in shells. The only similarity they had with Alex and Teena was that they walked on two legs. This adventure sure wasn't like one of those old 'B' movies where you could always tell an actor was camouflaged in a monster suit....