"There used to be a tree there," says Graham Cole, pointing to where a tree no longer stands. He is talking in the neatly kept grounds of Auschwitz, Nazi Germany's most notorious concentration camp.
"I would often use it when talking about what happened here. I would ask: `Imagine what that tree would have seen, what it would have witnessed.' But it has gone now."
Cole, one of a group of educators used by the Holocaust Educational Trust (HET) to teach British students about what took place in this infamous corner of Poland, recalls the tree as he discusses the challenges faced in ensuring the lessons from Auschwitz-Birkenau are never forgotten.
The point, he says, is that the sites of Auschwitz and Birkenau will continue to change as they slowly decay over time. The task then is to make sure that what happened there is kept alive in the minds of future generations.
On Sunday, what took place in Auschwitz-Birkenau will be remembered the world over as Holocaust Memorial Day marks the 68th year since the Red Army arrived at the gates of the camp.
With the passing of each year the experience of life within Nazi concentration camps moves further away from living memory. This is why the HET is given £1.5 million a year by the Department for Education to fly 200-strong groups of secondary school pupils from Britain to southwest Poland several times a year. The aim is to prevent the horrors of Auschwitz and the motivations behind them from dropping out of the consciousness of younger generations.
Facts in context
One such group is standing outside the entrance to Auschwitz on an unseasonably warm and sunny morning. In the bright sunshine, the camp has a discomforting resemblance to a theme park. Busloads of tourists with expensive cameras dangling around their necks mill about beneath the willow trees.
Even inside, when the gates adorned with the renowned words "Arbeit Macht Frei" loom into view, the camp still has the feel of a film set. It is a sense that is obviously felt elsewhere in the group as camera phones are snatched from pockets.
"Labour makes you free," says one boy, nudging his friend. The facts they have learned about the place trip almost absent-mindedly off the tongue.
Most of the group are well versed about the facts. Many are able to recall the numbers of people that were sent to their deaths in the camp, and others can recount how many more were killed in the concentration camps elsewhere in Europe.
But knowing there are piles of human hair in Auschwitz is not the same as seeing them. So much hair it fills a space from floor to ceiling. Piles and piles of shoes, including tiny pumps and plimsolls that would have once fitted tiny feet. And all the front door keys that were kept by so many of the victims before they were led to the gas chambers.
"If you think about it, it is the obvious thing you would keep," says Cole. "Nearly all of the people when they were ordered from their homes believed that they would one day return. So they kept hold of their keys."
The group is then shown the product of the Nazis' concentration camps, how they learned to turn a profit from death, such as by making cloth from human hair. "It got to a point where the Nazis began to see other human beings as a commodity," the guide says.
`Everyone should visit here'
Next we are taken via a short coach ride to Birkenau. The sheer scale of the site is breathtaking. As far as the eye can see are the brick chimneys that mark the spots where prison barracks once stood. A clutch of wooden huts have been rebuilt to give visitors a sense of what life was like in there. Originally built as stables for the Nazis' horses, the shacks soon became home for what was, on average, the remaining three to four months of a prisoner's life.
The pupils then make the long trudge - more than half a mile - to the far end of the camp and the gas chambers. They walk alongside the rail tracks, which have become the enduring image of the concentration camp and which carried so many of the camp's victims to their demise.
"Have you ever seen Schindler's List?" one girl asks her friend, taking in the converging tracks. "No," comes the reply. "I could never bring myself to watch it. Too depressing." The words flutter out of their mouths like butterflies, puncturing the grim silence with colour.
The hush returns as the group is guided through the building where the SS would decide who would live and work in the camp and who would be sent straight to their death.
We are then led to the gas chambers, which the Nazis attempted to destroy in a bid to remove any evidence of what they had done. The collapsed chambers resemble ancient ruins: sunken, grass-covered pits in the ground are half-covered by rubble.
The only real remains are some stone steps, leading down to a concrete trench where the inmates would have been forced to strip before being taken to the chamber where they were gassed with Zyklon B.
Just as in the rooms full of people's hair, shoes and suitcases, the teenagers are hushed into silence upon seeing the chambers. No pictures are being taken on camera phones.
"I had been to a concentration camp near Berlin but this is just massive. You read about it in books and you hear the numbers that were killed but it doesn't really hit you until you see it," says Ben Dupres, a Year 12 student from Ringwood School in Hampshire. "It is very emotional. It really does bring a lump to your throat."
Emmanuel Eghobamien, a student from Cranbrook School in Kent, acknowledges that the day was psychologically tough.
"I think everyone should visit here. It has been very draining emotionally but I think people need to see it to believe it. It is very moving," he says. "It has been a long day, but you can hardly say your feet ache when you know how much these people went through."
Night begins to fall and a vigil is held near the sites where an estimated 1.1 million people lost their lives. The pupils light candles, which they dot along those infamous train tracks, lighting their way home.
Sign of commitment
Student ambassadors working with the Holocaust Educational Trust (HET) were due to visit the deputy prime minister, Nick Clegg, at the Cabinet Office this week.
On Wednesday 23 January, the group of students were set to ask the Liberal Democrat leader to sign the HET's Book of Commitment, where signatories pledge to remember the Holocaust and to work to ensure that prejudice does not gain such a foothold in society.
Clegg joined the students on a visit to Auschwitz-Birkenau back in October, which marked the 100th school visit to the concentration and death camp organised by the HET.
On visiting Auschwitz, the deputy prime minister was asked to sign the visitors' book, in which he wrote: "Indifference and forgetfulness make hate, prejudice and genocide possible. That is why we must never forget the Holocaust and no visitor to Auschwitz today can."
Explore the history of the concentration camps using the TES Holocaust Memorial Day collection, a handpicked selection of free teaching resources, many of them from the Holocaust Memorial Day Trust.
Find lesson plans and resources on the pogroms, anti-Jewish laws and Nazi ideology in the TES Kristallnacht collection.