In October 1943, two glass jars were buried beneath an apple tree at 90 Grojecka Street in Warsaw. Inside each jar were hundreds of thin strips of tissue paper. On each strip was written a child's real name — and the name and address of the Polish family where the child was hidden.\n\nThe jars belonged to Irena Sendler, a 32-year-old social worker who had smuggled children out of the Warsaw Ghetto for three years. The jars were her insurance policy — if the war ended and she survived, she would be able to reunite each child with their true identity.\n\nThe jars survived the bombing of Warsaw, the Warsaw Uprising of 1944, and the Soviet liberation. When Sendler dug them up, the tissue paper was remarkably intact.\n\nBut the problem was heartbreaking. Most of the families who had sheltered children were dead — killed during the uprising, executed in reprisals, or deported to concentration camps. Most of the children's biological parents were murdered at Treblinka. The jars contained 2,500 names, but there was almost no one left to reunite them with.\n\nSendler spent the rest of her life trying. Some children were placed with surviving relatives. Some were sent to Palestine. Some were raised by Catholic convents and never learned they were Jewish until decades later.\n\nOne child, David Beizer, was 18 months old when Sendler smuggled him out in a potato sack. He was placed with a Polish family in Lublin and raised as a Catholic named Janek. He did not learn he was Jewish until age 54, when his adoptive mother died and left him a letter.\n\nBeizer eventually moved to Israel and became a psychologist. When he learned Sendler's story in the late 1990s, he wept. "Irena Sendler gave me my life twice," he told a reporter. "Once when she took me out of the ghetto, and again when she saved the paper that told me who I was."