Imagine you are watching a boat of eight rowers training for a big race in the middle of a lake. You hear the whistle blow, and puzzled by what you’re seeing, you ask the coach: “It doesn’t seem that the boat is moving”. To what the coach replies: “It’s because I haven’t explained you what’s going on in the boat. You can’t see it from here, but the first rower is really rowing hard. The five behind are splashing the oars so that the first chap thinks they are working hard. And the last two, well, they are rowing hard… but in the opposite direction. So you say the boat is not moving? Well, that’s what we call a good day, because usually it goes backwards.”
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