electricity and someone wants to explain the flow of currents. compares electricity with water. And because you understand water you get some idea about the behavior of electricity. Or an astronomer wants to give you some idea of the expanding space so he’ll use the metaphors of an expanding balloon with white spots on it. As you blow the balloon up, the white spots get further away from each other at the same speed as the balloon blows up. In neither case are we saying electricity is water or the universe is a balloon with white spots on it the same way the human being has always used images to represent his deepest ideas of how the universe works and what man's place in it is.  --style raw