:help:I have a high energy 18 month old un-neutered male border healer who need some mental exercises to get him more mental stimulation. He gets walked multiple times per day including at least one long walk, i.e, 2+ miles and sometimes 3.5 to 5 mi. on weekends. [ I know! He could use more.] He is very pleasant but also very possessive. He growls at and shows his teeth to other people in the family when they get near me, even though if I walk out of the room or especially the house he happily plays with the same people. Telling him no,very sternly, does not stop his behavior. I have to crate him and even then when I let him out, if people approach me he will growl and show teeth again if they do not back off. I need to get him to stop this and I need to give him more mental stimulation. Any ideas for appropriate mental exercises and/or training techniques would be most appreciated.