The last word–

BOVINE CHALLENGEHow long would it take an average cow to fill the Grand Canyon with milk?

Answers to this question varied

hugely, ranging from the pedants

who argue over the definition of

an average cow to defeatists who

claim the stench would be too great.

The order-of-magnitude experts won

the day, with a surprising number of

them calculating an answer similar

to the first one below – Ed

● Obviously the first job would be to divert the Colorado river, which would otherwise interfere with the process. Secondly, the canyon would need to be dammed to retain the milk. Thirdly, because this is a desert environment, huge refrigeration capacity will be required to prevent the milk turning to cheese. And finally, to prevent loss of liquid by evaporation, the canyon will need to be hermetically sealed.

So, preparation complete, let’s wheel in Daisy, the average cow. In the UK average milk yield per day per cow is in the range 15 to 20 litres. So let’s settle on 17.5 litres. The canyon is 446 kilometres long by an average of 16 kilometres wide and 1.6 kilometres deep, which gives a volume of about 10 million billion (1016) litres. So by simple division Daisy would take about 1.8 million million (1.8 × 1012) years to fill the

canyon. This assumes the canyon has a rectangular cross section; for a triangular cross section, the time would be halved.

Now, suppose you don’t want to wait 300 times the age of the planet for your canyon full of milk. Instead, you could divert the world’s entire milk production to the canyon. This adds another requirement – a milk pumping infrastructure of epic proportions – unless you choose to use dried milk, which would be cheaper to transport, and then rehydrate it with water from the river. The UN Food and Agriculture Organization estimates that global milk production in 2004 was 504 million tonnes, which is equivalent to 489 billion litres, giving an estimated fill time of only about 20,000 years – still a pretty long job. Jon White

Rampton, Cambridgeshire, UK

● It all depends upon the size of the tanker truck the cow chooses to drive, the time it would take to drive from the milk distribution point, the inflow and outflow of the tanker truck, the ability to change the absorption and evaporation rates of the milk, and the ability of said cow to effectively block the exit route of the Colorado river.

Other considerations, of course, would be does the cow work an 8-hour day, does she ever get a day off, or does she work 24/7?

In a tangential vein, what subsidies would the US government be giving to the dairy farmers? This could be the making of another watershed in reality TV.Bob Friedhoffer

New York City , US

SWEAT ON THE HOOFWhy, after going on a long run, do I only start to sweat profusely immediately after I have finished. Other runners I have spoken to also experience this.

● When sweat evaporates it creates a layer of saturated air close to the skin, which inhibits further evaporation. During a run, the movement of air relative to your body replaces this with fresh air, allowing evaporation to occur.Every gram of sweat that evaporates takes 2260 joules of body heat with it. When you stop running, the layer of saturated air builds up and the sweat does not evaporate. That creates the perception that you have suddenly started sweating profusely, and also raises skin temperature – which does, in fact, make you sweat slightly more.

Proof comes from the fact that the phenomenon will not occur when you stop running on a windy day.Shane Maloney

School of Biomedical and Chemical

Science

University of Western Australia, Perth

● You actually begin to sweat not long after you have started to run, once your muscles have settled into a working routine and you are using more energy than you would if you were resting. What you are experiencing is the wind-chill effect of your own movement: as a result, the movement of air over your skin and through your clothes wicks the moisture away from your body before it can build up. Try wearing a small backpack or taping a section of

something like plastic cooking film over an area of your chest. Sweat will build up here and not elsewhere, even after quite a short period of exercise.

When you stop running your muscles still have reserve heat to expel and this, combined with less air movement over your body, means that sweat will build up on the skin. If you warm down after a run, rather than stopping suddenly, the build-up of sweat may well not occur.

Relative humidity and ambient temperature also have a huge effect on how much sweat is produced. Higher temperatures plus high humidity leaves you more sweaty than cooler and less humid conditions. Try running through a forest on a sunny day after rain compared with open country in a breeze on a cooler day.Dave Banks

Wellington, New Zealand

● There is a second, minor effect beyond evaporative cooling at work here. When you stop exercising, your muscles stop working and generating heat, but your body’s “thermostat” is still set to a higher temperature. As it gradually resets to a resting state, you continue to produce sweat to cool yourself down. David Gibson,

Leeds, West Yorkshire, UK

Last Words past and present, plus a full list of unanswered questions, are available on New Scientist’s website at www.newscientist.com

“Does the cow work an 8-hour

day, does she ever get a day

off, or does she work 24/7?”

THIS WEEK’S QUESTIONA matter of tasteWhy do cooked foods taste different after they have cooled from the way they tasted when they were hot?Alan Parson

London, UK

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