hi i have a 4 yr old Border collie bitch ,she is trained an her behaver is well at times ,but she has a problem wiv grawing an showing her teeth at people who come to our house ,even when they give her fuss,she also graws at my 10 yr old son ,my dog plays in the garden wiv my son but when he goes to give her fuss she shows her teeth an im worried she mite bite some one or nip them ,what should i do to help her stop this ???