Harley does the same thing! He's 13 weeks, and LOVES rocks!!! Every time we go outside, he will find something in his mouth!!! He just puts them in his mouth plays with them!!! Consistency is the key. Usually, I tell Harley to "Leave it" in a stern voice, and he is getting to understand that he is not suposed to do that. If possible, take a ball outside with you, and when he goes to eat something he's not suposed to, replace the object you don't want him to have with one he can have (Ball, chew toy or something like that) Just a thought!