For some it is the prosthetic limbs, quickly made redundant as their wearers were singled out on arrival. For others, it is the clumps of hair, almost two tonnes of it behind a glass screen, greying not as a result of age but of the preservatives that stop it rotting. Others see it in the suitcases, a misplaced faith that the owners might need to pack again.
It is something else for David Dennis, though. "What gets me the most are the kids' shoes," he says. "I've seen lot of things like that in museums before, but it is ." he tails off for a few seconds, before resuming, "it is just the number, the volume."
The scale of the artefacts on display at Auschwitz is almost overwhelming, in keeping with a horror that is too much to comprehend. But amid the jumble of thousands of tiny shoes, it is the individual ones that stick in the memory: the little red sandal with the cork sole; the toddler's black leather slip-on; the orthopaedic shoe with two inch blocks on the toe and heel.
Mr Dennis has visited two other Nazi death camps - Dachau and Theresienstadt - but that hasn't prepared him for the sheer size of Auschwitz. "This is so much bigger. It puts it into perspective," he says.
The Holocaust Educational Trust has been taking sixth formers on day trips to Auschwitz for the past nine years, but this is the first time the charity has taken a planeload of teachers, about 150 from across the country. "When you are teaching it you have pictures and you have books, but when you come and see it for yourself, it brings a whole new dimension to your teaching," says Mr Dennis, head of history at King Edward VI Humanities College in Spilsby, Lincolnshire.
Just as on the trips for sixth formers, today's visit starts at the Jewish cemetery at Oswiecim, the Polish town that lent its Germanised named to the camp a few minutes away. Jews were in the majority at Oswiecim before the Nazis arrived, but by 1941 the entire Jewish population had been deported and the gravestones were being used as paving stones.
"Before they became victims, they had families and working lives and were ordinary individuals," says Shabana Marshall, the trust educator leading the trip. "They were caught up in events beyond their control, but we want to begin the narrative long before the Nazis were on the scene."
The trust runs about 14 trips to Auschwitz a year, and although a number of teachers accompany the sixth formers, Mrs Marshall says a teacher-only trip was arranged so more of them could experience the Lessons from Auschwitz project first-hand. When the teachers return to the UK, the trust will run a follow-up seminar looking at ways of putting their experience to practical use in lessons. "Teachers can be overwhelmed because they want to do the subject justice, but sometimes don't know where to start," she says.
The sheer weight of history is just one of numerous pitfalls awaiting unsuspecting teachers in educating classes about the Holocaust, says Tom Jackson, the trust's outreach co-ordinator. While drama can work brilliantly, he says asking pupils to imagine they are either an inmate or a camp guard can be dangerous territory. Similarly, teachers asking their pupils to write a diary of an inmate or a survival guide to the camp should also be wary. "No one knew how to survive - they were surviving on an hourly basis," he says.
"(These lessons) are done with the best intentions, but I would question the educational value of getting pupils to make stuff up when there is so much eye-witness evidence."
He cites the trust's own DVD of eyewitness accounts, Recollections, as suitable for the classroom, and says factual accounts should be preferred to fictional: he has reservations about the recent film The Boy in the Striped Pyjamas, which tells of a friendship between a camp commandant's son and a Jewish boy.
Natalie Hawkins - one of the teachers on the trip - has invited Holocaust survivors into her school, Caroline Chisholm in Northampton, to talk to the pupils, but as survivors age this sort of testimony will not be available to schools for much longer. Seeing the camps first-hand and sharing ideas with other teachers prompted her to come on the trip.
"From a practical point of view, the Holocaust can be quite difficult to teach, so being able to pass something on that I have seen myself will be a huge benefit," she says. "Personal experiences mean far more than something out of a textbook, so the more stories I can tell them, the more interesting it is to the kids."
Atiq Uddin, a history teacher at the Nottingham Bluecoat School and Technology College, plans to use his photographs of the trip to put together a series of lessons on the Holocaust. His school is moving away from setting homework and towards projects run on a half-termly basis. Work on Auschwitz could feed into projects on conflict resolution and diversity. "It is somewhere you read about and see on television, but seeing it is something else," he says. "To stand in a place like this that is drenched in blood gives you a sense of realism and the passion to try to convey it to the kids."
Many of the two-storey brick buildings at the main Auschwitz camp have been turned over to exhibitions, of photographs of life in the camps, models of the gas chambers, and the most treasured possessions of those who left their homes with what they could carry in a suitcase, little knowing they would be of no use in the camp: hairbrushes, pots and pans, shoe polish, the suitcases themselves.
A separate cabinet contains a pile of empty grey cans the size of paint pots, whose faded labels reveal they once contained poisonous pellets of Zyklon B.
The exhibition tells visitors that 1.3 million people were deported to Auschwitz, of whom 1.1 million died in the camps. The majority of the camp's inmates, about 1.1 million, were Jews, herded into Auschwitz on cattle trucks from all corners of the Nazi's short-lived empire.
When so few prisoners survived Auschwitz, there is a particular resonance to a building known as the Block of Death. At the end of a row of barracks, this was where those who tried to escape or broke one of the camp's many rules went to be punished. With no ventilation, the suffocation cell starved its inmates of air; the standing cell saw four prisoners forced to stand all night for nights on end in a telephone box- sized space. Outside is the courtyard where prisoners were shot, in the days before large-scale gassing was introduced.
"It makes it all very real," says Rhianon Selby, a history teacher at Barr Beacon Language College in Walsall. "Instead of an image in a textbook or on the internet, seeing those suitcases and the combs and all the everyday things makes me feel more passionate about teaching it, and if it's something I have experienced rather than read, it will enhance the pupils' experience."
Barr Beacon has sent sixth formers on the trip for the past two years, and for Mark Riches, director of humanities, this was a chance to appreciate what they had gone through. "You can talk about it in the abstract and refer to photographs, but once you have experienced something like this you can put that extra oomph in your teaching," he says.
Disturbing as they are, the exhibitions at Auschwitz made it hard to imagine the barracks as they would have been. Those difficulties fade at Birkenau, three kilometres away, where some of the single-storey wooden huts, the more familiar image of the camps, have been restored to their original condition.
The huts were originally stables, housing 52 horses each, before being shipped to Auschwitz and becoming home to 400 prisoners a time. Triple bunk beds, with six prisoners on each bunk, are packed tightly together. Next door is the latrine, a concrete strip with 200 holes above a trench. A place on the team cleaning out the latrines, the Scheissekommando, or shit group, was highly sought-after, providing a respite from the elements on the exposed plain of Birkenau.
"It's staggering to see how people can treat each other," says Jane Robson, assistant head of humanities at King Edward VI. The school invited survivors of the Rwandan genocide to talk to pupils last year, and Mrs Robson, who teaches citizenship and RE, says this sort of personal contact can have a huge impact on the children. "It is history, but you can relate the Holocaust to what is happening in the world today," she says.
Auschwitz is a visible symbol of the consequences of allowing extremism to flourish. For Kevin Egan, a teacher at St Wilfrid's Catholic High School in Sefton, Merseyside, seeing the camps at first-hand provides a powerful incentive to encourage children to get involved in society. "More people voted for the BNP than voted for the Nazis," he says. "We should encourage as many teachers as possible to come here."
At the far side of Birkenau is the sauna, where new arrivals were registered, stripped of their clothing and possessions, disinfected and issued with prison uniforms. Among these possessions were family photographs, some of which now form a poignant memorial.
Mrs Marshall says in the seminar that on their return they will look at ways of translating the experience to the classroom. "It is not about shocking or traumatising people; it is about how to apply the lessons to the modern world," she says.
Mr Dennis is already thinking about how he will use the experience in his Year 9 lessons. One idea is to put a pile of children's shoes on a table and ask his pupils for their thoughts. "Children have wonderful imaginations, and if you can get them to look inside people's minds and understand what it was like here, that can be very powerful," he says.
- The Holocaust Educational Trust publishes a range of resources including Recollections, a DVD of survivors' testimonies, Paul's Journey and Martin and Erica's Journey, accounts of individual survivors, and Lessons of the Holocaust. www.het.org.uk
- The Holocaust Memorial Day Trust also provides resources, including images, readings and poetry. www.hmd.org.ukresources
- The Holocaust Centre publishes books and CD-Roms for schools, including posters and comparisons with genocide in Rwanda and the Balkans. http:hcentrenew.aegisdns.co.uk.