Niddah 58 - Attribution
Shmuel made the following statement, "If a woman examined the ground of the earth and, finding it clean, sat on it, and then stood up and found blood on the ground, she is still ritually pure, since the Torah said 'Blood flowing in her flesh' - and not on the ground." The Talmud challenges Shmuel from many angles, because indeed, how is ground different from a garment? However, in the end it answers for Shmuel that the Sages, who established the laws in regard to stains, formulated it similar to other laws of ritual purity, and ground does not ever become impure.
If a woman finds a stain but can attribute it to any other possible source, she should; for example, if she slaughtered an animal or a bird, if she handled garments with bloodstains on them, or if she sat next to people who handed such garments. She may attribute it to her son or her husband, if they were bloodstained or handled blood.
There was an incident with a certain woman who came before Rabbi Akiva and said, "I found a stain." He asked her, "Perhaps you had a wound?" She replied, "Yes, but it healed." He then said, "Perhaps if you moisten it, it will bring forth blood?" She answered, "Yes," in he declared her ritually pure. He saw that his students were looking at each other and told them, "Why is this difficult in your eyes? The Sages who enacted the laws of stains did it with the provision to be treated leniently."
Art: Harry Watson - Young woman sitting upon rocks