Charlotte McDonald

That grass gets its water from rainfall.

The different methods of farming make a massive difference to the amount of blue water you have to pump out of rivers, lakes and aquifers to produce the cow food.

That figure we're looking at, that 15,000 litres of water are needed for each kilogram of beef, includes all types of farming and all the types of water.

It's a weighted average of the water use across all the different beef production systems the researchers analysed.

And 94% of that figure, more than 14,000 litres of water, is green water.

It's rain falling on grassland or other crops.

This, says Tim, makes it pretty useless if you're using the number to think about how good or bad eating beef is for the environment.

As with coffee in the rainforest, it's unclear whether it's a good or bad thing.

At the same time, the 15,000 litre claim also averages out the blue water part, which, as Tim said earlier, can range from less than 100 litres to many thousands.

The average is about 500 litres, but that hides the bit that potentially matters most in terms of the impact on water supplies.

All of which means you can't use the global water footprint to say that beef is bad.

And when Mesfin sees his numbers used like that, it makes him uncomfortable.

That's it for this week.

Thanks to Mesfin Meekinen, Mark Mulligan and Tim Hess.

If you've seen a number you think we should take a look at, email us on moreorless at bbc.co.uk.

Until next time, goodbye.

We're the programme that looks at the numbers in the news, in life and in surveys.

Yeah, often they're from surveys.

I'm Charlotte MacDonald.

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