"If only, when one heard That Old Age was coming One could bolt the door Answer 'not at home' And refuse to meet him!" (Anonymous, "Kokinshu" Imperial poetry anthology, 10th century)
Once upon a time there was a ruler who hated old people. Any of his subjects who lived past 70 he banished. A minister of the country loved his aging mother and could not bear to be parted from her. When she turned 70, he dug a secret underground chamber in his house and hid her in it. Years passed. One day a neighboring state sent two nearly identical horses — not as a gift but as a threat, for failure to answer a riddle connected with the horses would result in a deadly attack. The riddle was, Which of the two is the parent, which the offspring?
The ruler appealed to his minister, who promised to think of something. He knew who might have an answer — his mother, still in her secret cell. "She is old; she may have heard of something like this." Indeed she had. "Place grass between them," she advised. "The one who steps back and lets the other eat first is the parent."