I will be posting tips on training working dogs such as the Belgian Malinois, todays tip would be not to use tennis balls with your dog, there are a few reasons why you should not do this and one is the tennis ball is made for playing on asphalt therefore it is very abrasive, when a dog gnaws on the tennis ball it creates havoc on the enamel on the teeth, see below the photo of my dog max and the silver tooth, at this point all four canines have been done this way at a cost of over 5,000 and he will need more work., also if you know of anyone that has a dog that died of stomach cancer, I would look at the manufacturing of tennis balls and what happens when a dog ingests them.