Some of the molecules from a cup of water which somebody drank 100years ago when he took a break from working in the fields became a part of him, 1000 years ago, and later, some of them became a part of a glass of wine for a wedding reception 800 years ago, and some of those molecules became a part of scoop of water for someone who ran a long distance to deliver a message and then became his sweat 600 years ago, and later, some of the same molecules became a part of a sumi-ink for painting 400 years ago, which then became a part of the water flowing through a steam engine 200 years ago, which became a drop of water in a bath water for a model who worked for Renoir 100 years ago. When I drink a cup of water, I rarely think of its history. However, as scientists agree, water molecules keep on cycling through the atmosphere.