Birthmark

Why Chinese babies like to have birthmark look like bruise on their bottoms? When Yaya first attended school, the teachers became so concerned one day when they noticed her bottom with the blueish marks, and very politely asked whether it was their fault to have the baby somhow fell without noticing it... Now, in the new school, the director and Peter's teacher, again, took it very seriously.. We will need a note from the pediatrician to confirm that it is birth mark that on Peter's bottom...