Charlotte McDonald

As they grow, plants absorb water from the soil through their roots.

This is sucked up through the plant to the leaves where it evaporates.

It's a process called transpiration.

This might sound a bit technical, but for the work Tony Allen was doing, the importance of the water required to grow plants was pretty obvious.

In very dry places, water is a precious resource.

The water you use for irrigation is water you can't use for something else, like drinking or sanitation.

This water has a clear environmental cost.

The idea of virtual water was taken on by Arjen Hoekstra and expanded.

He developed a method for figuring out how much water a food crop or animal used, wherever it was in the world.

But to point out the obvious, not all parts of the world are arid drylands where there's little to no rain.

In this case, if you think a higher water footprint is a bad thing, then you'd be saying that chopping down rainforests for coffee plantations is good.

What's more, the water we're talking about falls as rain.

And to put it mildly, rain is not in short supply in a rainforest.

This is Tim Hess, a professor of water and food systems at Cranfield University in the UK.

Green water is rain.

Blue water is pumped out of rivers and lakes.

There's a third category, grey water, which is the water that's needed to dilute any pollution, like fertiliser runoff.

Right, back to the beef.

If we look at beef production and this 15,000 figure... The majority of the water is associated with producing feed.

In the UK, cows mostly stand around in fields eating grass.