"Well," Harry said, "look at it this way: Suppose you were an intelligent bacterium floating in space, and you came upon one of our communication satellites, in orbit around the Earth. You would think, What a strange, alien object this is, let's explore it. Suppose you opened it up and crawled inside. You would find it very interesting in there, with lots of huge things to puzzle over. But eventually you might climb into one of the fuel cells, and the hydrogen would kill you.