I posted in a Facebook parenting group I sometimes loiter in, and asked mothers to share the strangest things they had kept from when their children were young. “I saved my son’s foreskin,” said Casey Revkin, 42, chief financial officer of an immigration nonprofit and a mother of one. “I saved two pieces of paper that my daughter stabbed with a pen about 100 times,” said Ahna Tessler, a 40-something comedian, photographer and mother of twins in Manhattan. 