Security is sleeping in the back seat of the car...When you're a little kid, and you've ben somewhere with your Mom and Dad, and it's night, and you're riding home in the car, you can sleep in the back seat...You don't have to worry about anything...Your Mom and Dad are in the front seat, and they do all the worrying...But it doesn't last! Suddenly, you're grown up, and it can never be that way again!

解释：在人活动范围内的水就污浊，在野外无人地方的水就清洁。同为一样的水，都是从天边开源，有的混浊有的清洁，这是它们所处环境使它们这样的。