So, imagine I have a plank about two feet wide and 30 feet long and I'm going to walk on it, and you see I can walk on it without looking, I can go back and forth and I can jump up and down.
设想一下 我有一块2英尺宽 30英尺长的木板 我可以在木板上踱步 而不需惊慌 我可以往前 往后 可以跳上跳下