For more than 20 years, Harry Bibring has been telling his story to thousands of pupils. He's been a living witness.
He's talked about what he saw as a Jewish child in the 1930s when the Nazis took over his home city of Vienna in Austria. He last saw his parents in March 1939 when he caught a train, part of the Kindertransport, which brought Harry and his sister to England.
Such survivors of the Nazis have visited many schools, sharing their memories, passing their first-hand stories from one generation to the next.
But there is no escaping the passing of time and the remaining survivors are now getting frail. This is the 70th anniversary of the end of the Holocaust and there won't be many more big anniversaries when teenagers will be able to hear from people who saw these events with their own eyes.
These are the last witnesses and their stories will slip from living testimonies to recorded history.
BBC News - Last witnesses of the Holocaust