This strange and counter-intuitive effect was first observed by the ancient Greek philosopher and was made famous in recent times by a Tanzanian school student called Erasto Mpemba. He noticed that the sugared milk he was using to make ice cream froze more quickly if it started out hot. But what is behind the so-called "Mpemba effect"?

According to Jonathan Katz of the Washington University in St Louis, it's all to do with solutes. "You have to ask yourself: what does heating do to water that makes it easier to freeze?" he says. "The answer is that it precipitates out solutes."

The solutes Katz has in mind are calcium and magnesium bicarbonate, which make most drinking water "hard". When the water is heated, these precipitate out to form the solid scale that "furs" up the inside of a kettle.

Water that has never been heated still contains these solutes. As it freezes, ice crystals form, and the concentration of solutes in the remaining water becomes ever higher - up to 50 times as high as normal. This lowers the freezing point of the water, just like salt sprinkled on a road in winter. "The water therefore has to cool further before it freezes," says Katz.

There is a second, related effect that hampers the freezing of water that has never been heated. The lowering of the freezing point reduces the temperature difference between the liquid and its freezing surroundings. "Since the rate at which heat is lost from the water depends on this temperature difference, water that has not been heated has greater difficulty losing heat," Katz says.

Katz claims that the two effects combined can perfectly explain why water that has been heated freezes more quickly than water that hasn't. And he makes a prediction that experiments should be able to verify: that the Mpemba effect should be more marked the "harder" the water. "This may explain why not everyone sees it," he says. "Some people are using soft water."

"Katz's analysis of the Mpemba effect is deeper and more rigorous than anything else on the subject," says Richard Muller of the University of California at Berkeley. "He has come up with a simple yet - I believe - correct way to look at a complex phenomenon."

Katz, who worked out the details of the Mpemba effect while adjudicating a student exam, is waiting for someone to do the experiment to test his theory. "It's not difficult but it's not trivial either," he says. "I think it would take a couple of months to do it right."

From issue 2554 of New Scientist magazine, 03 June 2006, page 10

The suspects were charged with owning and distributing child pornography via the Internet, police said.

In Spain, police searched several cities for the 10 suspects.

"All the people who were operating in our country had a high level of knowledge of information technology and a high level of education. They included several university professors," the police said.

Police were investigating a further 57 suspects in 28 countries, according to the statement.

"We felt that perhaps we could compromise our principles but provide ultimately more information for the Chinese and be a more effective service and perhaps make more of a difference," Brin said.

Brin also addressed Internet users' expectations of privacy in an era of increased government surveillance, saying Americans misunderstand the limited safeguards of their personal electronic information.

"I think it's interesting that the expectations of people with respect to what happens to their data seems to be different than what is actually happening," he said.

Google has battled the U.S. Justice Department in court seeking to limit the amount of information the government can get about users' Internet searches. It also says it has not participated in any programs with the National Security Agency to collect Internet communications without warrants.

Google's free e-mail service is among the Internet's most popular.

Brin visited Washington to ask U.S. senators to approve a plan that would prevent telephone and cable companies from collecting premium fees from companies such as Google, Microsoft and Yahoo! for faster delivery of their services. Brin, dressed casually in jeans, sneakers and a black sport jacket, said he wasn't sure whether he changed any lawmakers' minds.

Google's China-approved Web service omits politically sensitive information that might be retrieved during Internet searches, such as details about the 1989 suppression of political unrest in Tiananmen Square. Its agreement with China has provoked considerable criticism from human rights groups.

"Perhaps now the principled approach makes more sense," Brin said.

The Paris-based group Reporters Without Borders said Tuesday that Google's main Web site, http://www.google.com, was no longer accessible in most Chinese provinces due to censorship efforts, and that it was completely inaccessible throughout China on May 31.

Brin said Google is trying to improve its censored search service, Google.cn, before deciding whether to reverse course. He said virtually all the company's customers in China use the non-censored service.

"It's perfectly reasonable to do something different, to say, 'Look, we're going to stand by the principle against censorship and we won't actually operate there.' That's an alternate path," Brin said. "It's not where we chose to go right now, but I can sort of see how people came to different conclusions about doing the right thing."

"We think that by the end of this year the Italian military mission will end in Iraq."

It is the first time a minister from the newly elected centre-left government has given a timetable for the pullout of Italy's 2,700 troops. The Italian prime minister, Romano Prodi, was recently elected on a platform of withdrawing troops from Iraq swiftly but until today his government had not discussed timing.

The previous government, under the staunch US ally Silvio Berlusconi, also had vowed to complete the pullout by the year's end. Speaking during a visit to Baghdad to discuss the plans with Iraqi leaders, Mr D'Alema said: "We have a voters' mandate: the troops are coming back home."

Mr D'Alema met with his Iraqi counterpart, the foreign minister Hoshyar Zebari, shortly after his arrival in Baghdad, and was scheduled to meet the prime minister, Nouri al-Maliki, the foreign ministry in Rome said.

Mr Zebari said the Iraqis would be ready to take over responsibility for the south-eastern area where the Italians are based.

"This withdrawal will not begin suddenly but will be gradual," he said during a joint news conference at the Iraqi foreign ministry. "We have a security plan to transfer the security tasks from the Italian forces to the Iraqi forces starting at the end of this month."

Mr Zebari also plans to meet with the US secretary of state, Condoleezza Rice, in Washington next week to discuss the withdrawal.

The announcement comes two days after an attack on an Italian military convoy in southern Iraq that killed a soldier and wounded four others. But Mr D'Alema said the pullout was strictly a political decision.

Mr Prodi, who took office last month after elections that ousted Mr Berlusconi, also told parliament on Tuesday that the attack would not speed up the pullout plans.

In the next few weeks, Mr Prodi and his cabinet will devise a more detailed timetable for withdrawal. Some of the more leftwing parties in his broad alliance are pressing for Italy's troops to be back by the summer.

Last month Mr Prodi made it clear he intended to pull out the third-biggest contingent in the military coalition at the earliest possible opportunity.

In his first policy speech since being sworn in, he criticised Anglo-American policy, calling the invasion of Iraq a "grave mistake" and branding the allied military presence an "occupation". The departure of Italian troops from Iraq would further weaken a coalition that has been hostage to anti-war sentiment in Europe as well as financial constraints.

Ukraine withdrew all of its troops from Iraq at the end of last year, and Poland, which was once one of the largest military contributors, has also cut its contingent. Several other smaller forces have also been withdrawn.

The Italian withdrawal is part of an overall reduction in the US-led coalition force planned for this year. The US has announced it will reduce its 140,000-strong force, and Britain, which has the second biggest contingent, is to cut back its 8,500-strong force.

Italian forces have suffered 32 deaths in Iraq. The Italian contingent is based in Nasiriyah, about 200 miles south-east of Baghdad. The Italian troops were sent in by Mr Berlusconi to help rebuild Iraq after the 2003 ouster of Saddam Hussein.

A criminal on the run hijacked the helicopter Sunday to get his brother out of prison, police said. Vassilis Paleokostas, 40, who was serving a 25-year sentence for kidnapping and bank robbery, and an Albanian convict escaped.

"The guards thought it was a surprise inspection by ministry officials and did nothing," a police official said.

The helicopter pilot, who said he was forced at gunpoint to undertake the mission, flew the inmates to a nearby cemetery and they made their escape on motorbikes. Greek police have launched a manhunt for the convicts.

SHADY HILLS - Hours after the killing of Gilbert Wood, as detectives pulled clues and women wept, Daniel Boyle stood near the crime scene and spoke ill of the dead.

Everyone else said Wood, 81, was harmless and benevolent. He had no criminal record in Florida. But here was Boyle, his neighbor from across the street, describing Wood's double-wide as a drug-dealing haven.

"Hate to say it," said Boyle, 42, "but he probably got what he f---in' deserved."

On that day, May 26, the comments could have been dismissed as idle slander. But a search warrant that the St. Petersburg Times obtained a copy of on Monday put them in new perspective. The Sheriff's Office said it found what appeared to be bloodstains on the seat of Boyle's silver Toyota pickup, as well as a crescent wrench wrapped in a moist towel that seemed to match a wound on Wood's head.

Boyle remained free Tuesday morning, however, and he told a Times reporter the warrant was meaningless.

"Pasco County sheriff's department," he said, "can kiss my a--."

He said his wrench never struck Wood, and he maintained that tests would show the blood in his truck was his own - a casualty of yard work.

"I'll show you the plant I cut it on," he said, pointing east along his barbed-wire fence. "The damn cactus right there at the end."

Boyle said he served four years in the U.S. Marine Corps, and his 1.2-acre Rockledge Avenue compound resembles a fortress. State records show that he has lived in Florida since at least 1989 and has never been charged with a crime.

His jungle-like yard brims with bamboo and saw palmetto. Rusty nails point skyward on his fence posts, and a metal gate blocks his driveway. Boyle said the neighborhood is rife with drug dealers. So did his next-door neighbor to the east, Kathleen Jordan, the woman who found Wood's body.

That is one of the few things they agree upon. Jordan told the Times she took Wood to the bank a few days before he died. He withdrew several thousand dollars for a down payment on a new home. When he went to pay Boyle for digging up his palm trees, according to Jordan, Wood accidentally showed him the envelope thick with cash.

Later, according to Jordan, Boyle told her this:

"Man, it'd be so easy to break in the back door and bop him over the head and take it all."

She said she thought he was joking. But on May 26, she found Wood on the floor of his house, covered in stab wounds. The money was gone.

Wood never made it off Rockledge Avenue. But Jordan said she will. She said she is selling her house. She said she is afraid to live next to Boyle. She said his yard is full of booby traps.

She pointed across the fence onto his property, where a homemade wooden watch tower stands in the back yard, crowned with a black flag to commemorate prisoners of war. She said he likes to sit up there, looking down at her swimming pool. She said her husband called Tuesday morning and told her to go somewhere safe.

It was nearly noon when Jordan loaded two young children into a dusty pickup truck and drove west on Rockledge Avenue.

She passed the barbed-wire fence, the blade-sharp vegetation. Boyle was in there somewhere. She did not slow down. The engine roared as she vanished over the hill.

Suppose, just suppose, that one or more of the 17 charged yesterday with terrorism is innocent.

This is not the common assumption. I suspect most Canadians assume that Ontario was in great danger from terrorists, that police nipped this danger in the bud and that all of the 12 adults and five young people they arrested are guilty.

All of which may be true. Terrorists do exist. There is the terror we don't think about, committed by nation states under the rubric of security sweeps or targeted reprisals. And there is the terror we do think about, the terrorism of misguided individuals, loons, right-wing militias or Al Qaeda and its Islamist acolytes.

Militant Islamists have committed outrages in the United States, Indonesia, Spain and Britain to counter what they see as the crimes of these countries against Muslims. There is no obvious reason to assume that similar criminals won't try the same thing here.

All of which is to say that the Mounties may be absolutely correct when they say they stopped the 17 from using homemade detonators and three tonnes of fertilizer to blow up as yet unspecified targets in southern Ontario.

There may indeed have been a terrorist conspiracy that involved what the RCMP assistant commissioner Mike McDonell yesterday referred to as "training areas," where militants tramped about in big boots, cooked on outdoor barbecues, built bombs and used a wooden door for target practice.

That's the implication from the evidence shown to reporters yesterday: five pairs of boots in camouflage drab, six flashlights, one set of walkie-talkies, one voltmeter, one knife, eight D-cell batteries, a cellphone, a circuit board, a computer hard drive, one barbecue grill, one set of tongs suitable for turning hot dogs, a wooden door with 21 marks on it and a 9-mm handgun.

Or it is possible that the only thing that these bits of evidence prove is that a group of young men went somewhere where they tramped around in big boots, cooked on barbecues, played soldier and generally acted like jerks - which young men are occasionally wont to do.

The three tonnes of ammonium nitrate allegedly purchased was, as McDonell said, three times the amount used in the Oklahoma terror bombing of 1995.

But, as he also said, farmers routinely buy three tonnes of ammonium nitrate "every day." They use it for fertilizer, not bombs.

In short, we don't know much yet about what these men and boys were trying to do. We don't know if this series of arrests, called Operation O-Sage by the Mounties, pre-empted the kind of actions that in the United Kingdom led to last year's bombing of the London subway by otherwise unremarkable young Britons.

That's one possibility. It's certainly the explanation favoured by Prime Minister Stephen Harper, who yesterday praised the police.

Another is that this is a reprise of the infamous 2003 Project Thread fiasco, in which RCMP and immigration officials accused 23 Muslims of terrorism only to acknowledge later that at most the men were guilty of minor immigration fraud.

Still another possibility is that this may turn out to be Canada's version of the 2004 Virginia "paintball" trial, in which one man was sentenced to life and another got 85 years.

In that controversial case (even the presiding judge complained the outcome was unfair), nine Muslim men were convicted of participating in terrorist training - the main evidence being that they had played paintball in the woods outside Washington.

What we do know about Operation O-Sage is that the RCMP, as well as the Canadian Security Intelligence Service, have been tracking the suspects since 2004. We also know that at least some of their neighbours knew police were watching them. Presumably, some of the suspects did, too.

If the alleged conspirators knew they were under surveillance, it seems odd that they continued along merrily with plans to make explosives.

But perhaps they are not bright terrorists. Or perhaps they are not terrorists at all.

With luck, we will get these answers at trial. This time at least, Canada has chosen to deal with alleged terrorists in the proper way, by charging them with criminal offences and allowing the case to come to court - in Canada.

For too long, the government's preferred option was to let others handle our problems. In 2002, CSIS agents escorted alleged Canadian terrorist Mohamed Mansour Jabarah across the border so he could be arrested by the FBI and convicted in a secret trial. Later that year, the RCMP co-operated with the Americans to have them arrest Canadian suspect Maher Arar in New York (he was later transferred to Syria to be tortured).

Five other alleged terrorists are simply being detained without charge under Canada's very elastic immigration act until they can be deported.

So, in this context, the 2004 decision to charge Canadian Mohammad Momin Khawaja for terrorism and yesterday's unrelated decision to charge the 17 are welcome. At least the accused aren't being sent to Syria.

During the next few days, much will be written and broadcast on the 17. Their lives will be re-examined through the prism of the arrests as reporters try to retrace the steps that allegedly led them to violent jihad. Unnamed security sources will leak details designed to bolster the police case. Families and friends will proclaim the innocence of those charged.

Take it all with a grain of salt. We know that police arrested people. We know they seized some materials - all legal - that can be used to make explosives. So far, we don't know much else.