When Canadian science graduate Christopher Charles visited Cambodia six years ago he discovered that anaemia was a huge public health problem.
In the villages of Kandal province, instead of bright, bouncing children, Dr Charles found many were small and weak with slow mental development.
Women were suffering from tiredness and headaches, and were unable to work.
Pregnant women faced serious health complications before and after childbirth, such as haemorrhaging.
Ever since, Dr Charles has been obsessed with iron.
Anaemia is the most common nutritional problem in the world, mainly affecting women of child-bearing age, teenagers and young children.
In developing countries, such as Cambodia, the condition is particularly widespread with almost 50% of women and children suffering from the condition, which is mainly caused by iron deficiency.
The standard solution - iron supplements or tablets to increase iron intake - isn't working.
The tablets are neither affordable nor widely available, and because of the side-effects people don't like taking them.
Lump of iron
Dr Charles had a novel idea. Inspired by previous research which showed that cooking in cast iron pots increased the iron content of food, he decided to put a lump of iron into the cooking pot, made from melted-down metal.
His invention, shaped like a fish, which is a symbol of luck in Cambodian culture, was designed to release iron at the right concentration to provide the nutrients that so many women and children in the country were lacking.
The recipe is simple, Dr Charles says.
"Boil up water or soup with the iron fish for at least 10 minutes.
"That enhances the iron which leaches from it.
"You can then take it out. Now add a little lemon juice which is important for the absorption of the iron."
If the iron fish is used every day in the correct way, Dr Charles says it should provide 75% of an adult's daily recommended intake of iron - and even more of a child's.
Trials on several hundred villagers in one province in Cambodia showed that nearly half of those who took part were no longer anaemic after 12 months.
In a previous trial, published in the European Journal of Public Health, participants started using water from wells after a few months, because of drought, which was contaminated by arsenic.
Arsenic blocks the uptake of iron so it looked like the fish had stopped working.
'Better than tablets'
Prof Imelda Bates, head of the international public health department at Liverpool School of Tropical Medicine, says the iron fish is a welcome development.
"These sort of approaches are so much better than iron tablets, which are really horrible.
"If it's something that is culturally acceptable and not too costly, then any improvement to anaemia levels would be of great benefit."
Around 2,500 families in Cambodia are now using the iron fish and the Lucky Iron Fish company, set up by Gavin Armstrong, has distributed nearly 9,000 fish to hospitals and non-governmental organisations in the country.
What pleases Dr Charles most is the fact that villagers appear to have accepted the smiling iron fish, which is 3in (7.6 cm) long and weighs about 200g (7.1 oz).
One woman and her daughter, who are part of a current trial in Preah Vihear Province, told the BBC they would use it during cooking.
"I'm happy, the blood test results show that I have the iron deficiency problem, so I hope will be cured and will be healthy soon.
"I think all the people in Sekeroung village will like the fish, because fish is our everyday food."
Scale of anaemia
The World Health Organization estimates that two billion people - over 30% of the world's population - are anaemic, mostly due to iron deficiency.
It says stopping iron deficiency is a priority - for individuals and countries.
"The benefits are substantial. Timely treatment can restore personal health and raise national productivity levels by as much as 20%," it has said.
And it emphasises that it is the poorest and most vulnerable who stand to gain the most from its reduction.
But there are other forms of anaemia. It can also be caused by vitamin B12 and A deficiencies, parasitic infections, such as malaria, and other infectious diseases.
That is when it gets complicated, says Prof Bates.
"Anaemia is a multi-factorial problem. It's the end product of many different health issues.
"And measuring whether people have enough iron or not in their bodies is very difficult in developing countries," she said.
As a result, she says, knowing how many people really are iron deficient isn't easy to work out.
In those with iron-deficiency anaemia, the cause is often poor diet. And that's the case in Cambodia, Dr Charles says.
"They have a really poor diet - a big plate of white rice and maybe a small cut of fish.
"That's their two meals a day. And it's just not meeting their nutritional requirements."
What's missing from their diet are iron-rich foods, particularly red meat. Green leafy vegetables, such as spinach, are not as rich in iron and mustn't be overcooked if they are to offer any benefit at all.
The Lucky Iron Fish project has a plan to get fish to every part of the world that needs them, including countries like Canada, the US and Europe.
So should everyone be putting recycled metal car parts in their soup?
According to the experts, there is no reason not to - although levels of anaemia are far lower in developed countries, and there is easier access to iron-rich foods which can make all the difference to pregnant women and vegans, for example.
We could all eat iron filings instead, of course, but they wouldn't taste half as nice.
What does iron deficiency do to the body?
Iron deficiency anaemia is a condition where a lack of iron in the body leads to a reduction in the number of red blood cells.
Iron is used to produce red blood cells, which help store and carry oxygen in the blood.
If there are fewer red blood cells than normal, your organs and tissues will not get as much oxygen as they usually would.
This means you can suffer from tiredness, shortness of breath, heart palpitations and a pale complexion.
If left untreated it can make people more susceptible to illness and infection.
Pregnant women and children are particularly vulnerable. Anaemia is thought to contribute to 20% of all deaths during pregnancy.
Source: World Health Organization