We take water for granted in the West. According to statistics from the charity WaterAid, in the developing world nearly a billion people - one in seven of the world's population - are at the mercy of dirty, diseased water and the misery it brings.
But how do you convey this message to children who innocently turn on a tap without a second thought? I explained it to my Year 5 class like this: 97 per cent of the Earth's water is saline - a fact I demonstrated using a large bowl and a clear 30cl water bottle. I poured 10l of water into the bowl (representing the world's total), then filled the 30cl bottle to represent all the fresh water in the world. Next I poured two-thirds of the water in the bottle into another container, explaining that this very roughly represented unreachable fresh water, such as that underground, in icebergs or in glaciers.
In other words, as I explained to an amazed class, the remaining one-third in the bottle is all the water we have to drink. And that has to be shared by about 7 billion people.
I took that water and split it into two glass jugs, explaining that one represented the drinking water of developing countries and one of wealthier countries in the rest of the world, such as the UK.
A lot of the water we drink comes from rivers, but I explained that many of these are contaminated with cholera, dysentery, typhoid and worm-based infections. (Explanations of these diseases and all their gory consequences can be found at www.wateraid.org.uk) So, I asked my class: if the water in our taps comes from rivers, why is it so clean? The children were invited to make the water dirty, using soil, sand, dead insects and even raisins to represent poo (they loved this bit). They were then dared to drink the water. Of course, no one did.
But I filtered one jug of dirty water through filter paper and a sieve and boiled it once it looked clear. One more sieve after it had cooled and it was clean enough to drink - which is what I did. In the other jug, the soil and sediments had sunk to the bottom, but food matter was dissolving and floating on the top. Which water, given the choice, would the children rather drink? Here, really, is the learning. My class had a choice. But millions of children don't.
Clean water shouldn't be a luxury, a privilege or an indulgence, it should be a right. Charities such as WaterAid raise money for villages in the developing world to have access to it. So raising awareness in school is among the most profound teaching that can be done in a primary setting.
Chris Fenton is an associate headteacher in the North West of England
WaterAid London has shared resources that help to teach pupils about sanitation issues around the world.
Explore water issues using a range of interactive resources from TESiboard.