My TA gave this analogy of a guy with a machine gun. He drew two boundaries. He said, 100 bullets pass through both boundaries per second. So in the space between the two boundaries, there is a zero flux. (?) But if there is a guy standing in between the two boundaries and he gets shot with three bullets per second, then there is a nonzero flux because there is not an equal amount of bullets coming passing through each boundary. (?) What does it mean that there is a zero or nonzero flux between the two boundaries? I thought that flux was the rate a which something passes through a unit area. I don't understand the idea of a flux in some sort of space. I understand the ideas of flux at each of the boundaries just not when you throw the guy in the middle. He also mentioned a river analogy that he said was simpler, buthe didn't explain it to me. Anyone know what analogy this is?