I looked out the window at the grey sky today and imagined a society in the future in which the sky is always grey, due to pollution or fog or something of the like.  Then I had a beautiful thought - perhaps people would begin to paint their ceilings blue in remembrance of the vibrant color of the sky on a clear day.  And perhaps, eventually, everyone would have blue ceilings. Every ceiling would be blue.  After a while, children would not know why all ceilings are blue.  Nobody would remember what the sky's color was originally, but at the same time it would be there constantly, right above their heads.