Science Puzzle
How Old Is the Water in Your Glass?
Fill a glass with water from the kitchen tap. It might have fallen as rain a few weeks ago, then flowed down a river into a reservoir, through a treatment works and along the pipes to your home.
But how old is the water itself?
The Answer
Billions of years old. Its atoms are older than Earth, and some of the water molecules themselves may be older than the Sun.
Earth doesn't make much new water and loses very little to space, so the same water goes round and round. It evaporates from the sea, falls as rain or snow, freezes into glaciers, runs down rivers and passes through plants and animals, again and again. A molecule in your glass has almost certainly been through clouds and oceans countless times. It has quite possibly been through a dinosaur.
Living things do split and rebuild some water. Plants break water molecules apart in photosynthesis, and your own body makes a little water when it uses food for energy. But the atoms themselves are far older. The hydrogen atoms were made in the Big Bang, about 13.8 billion years ago. The oxygen atoms were made inside stars that lived and died before our Sun was born.
A study published in 2014 suggested that up to half of the water on Earth formed as ice in the cold cloud of gas and dust that the Solar System grew from, before the Sun had even started to shine. Your glass of tap water could be holding molecules older than the Sun itself.
The principle: The water cycle and the conservation of matter. Matter is not used up but recycled, so the water on Earth today is largely the same water that has circulated for billions of years.