Cao Cao heard that Sima Yi was not only ambitious, but also had a "lang gu" () appearance, so he wanted to test and see if it was true. One day, he ordered Sima Yi to walk in front of him and then made him look back. Sima Yi purportedly turned his head 180 degrees to look back. Cao Cao also once dreamt of three horses feeding from the same trough and he felt disturbed, so he warned Cao Pi: "Sima Yi won't be content with being a subject; he will interfere in your family matters." As Cao Pi was on good terms with Sima Yi, he often protected and shielded Sima Yi from criticisms. Sima Yi also took great care to create an image of himself as a diligent and faithful subject in front of Cao Cao to reduce the latter's suspicions of him.