A countryman from the state of Lu sent to king Song Yuangong two knots that were difficult to untie.
The king posted a notice to find people with wisdom to undo the knots, but nobody could undo them.
Later one of Er Shou's students untied one of them but could not undo the other one. The student thereupon claimed, "The knot could impossibly be untied." The king sent people to check what he said with the countryman from Lu.
The man from Lu said, "He is right. This knot cannot be untied. It was not him that tied this knot. And he can tell that this knot cannot be untied. He surely is more clever than I am."