In facial palsy, the corners of the mouth deviate to the opposite side because the expression muscles on the paralyzed side are dysfunctional and cannot pull the corners of the mouth well when the patient smiles or shows teeth, while the healthy side has normal pulling function, which will cause the corners of the mouth to deviate to the healthy side because of the imbalance of the forces acting on both sides. In central facial palsy and peripheral facial palsy, it is important to differentiate: 1. If it is central facial palsy, damage to the left cerebral hemisphere occurs, facial palsy appears on the right side of the face, and the corners of the mouth will deviate to the left when showing teeth; 2. If it is peripheral facial palsy, facial nerve paralysis appears on the left side, and the corners of the mouth will deviate to the right when showing teeth. It should be noted that in peripheral facial palsy, the facial nerve paralysis is on the ipsilateral side, and in central facial palsy, the facial nerve paralysis is on the contralateral side.