They are all old enough that no explanation is needed. The youngest is almost thirteen and the oldest seventeen going on eighteen. It's the seventeen year old that has the biggest problem with it. Go figure. I do tell them it is perfectly natural, he is a dog after all.
Ten years and under is tough. I think it depends on their age and if they are seriously asking why he does it because they want to understand what he is doing. Then I've always believed in telling them the truth. I like them to come to me for answers. Start simple and see if they have another question. Something like dogs do that when they want to have puppies. That might be enough. In my own experiences it was more of a big deal to me than it was to them. If a four year old asks you could probably just tell them he is just being silly and that might be the end of it. I don't know if that helps but it is a tricky question.
