A Riddling Tale 谜语童话

Three women were changed into flowers which grew in the field,
but one of them was allowed to be in her own home at night. Then
once when day was drawing near, and she was forced to go back to
her companions in the field and become a flower again, she said to her
husband, "If thou wilt come this afternoon and gather me, I shall be set
free and henceforth stay with thee." And he did so. Now the question
is, how did her husband know her, for the flowers were exactly alike,
and without any difference? Answer: as she was at her home during
the night and not in the field, no dew fell on her as it did on the others,
and by this her husband knew her.