After wisdom tooth extaction, when healing takes place, the patient is advised to stretch is his mouth because sometimes due to muscle trismuss, there is difficulty in opening the mouth afterwards. In stretching of mouth you donot have to wide open the mouth but open the mouth till pain is there and donot go beyond that. If you can easily put your 3 fingers in mouth that means the opening is normal and if it is less then you have to try to open more and more gradually till you are able to put 3 fingers in the mouth. For more info on tooth extraction aftercare you can read this article.... http://www.identalhub.com/article_do's-and-don'ts-after-tooth-extraction-153.aspx