my 18 mth border terrier keeps nipping my 9yr old springer only last thing at night ging out and when they first come off the lead for a walk, she doesnt do it to the other dogs and doesnt do it to anyone elses dog its just him . they get on fine in th house and play fight where she doesnt nip at all in fact when she play fights she has their leg in her mouth but she doesnt hurt them(including him) so she knows not to bite but when she does it to him on theses times she does make him yelp. i do hold her back at the night time to make sure the other dogs go out first then i allow her out but she runs straight up to him to nip his ear until i shout ahah at her then she stops but she wont stop when ive let her off the lead. anyideas how to stop her or why she does it to him