Grimms' fairy tales

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."