"How many miles does a man have to walk before you can call him a man?"
There is no number of miles that determine if you are a man or not, and the question isn't really talking about walking, its about what do you have to do before you reach a level of wisdom or maturity where understand what is really happening. At least thats how I interpret that. There are different interpretations of this and if you have a well reasoned answer that makes sense to you then you are also correct.
However, if you decide that the answer is 10 you are missing the point of the question.... do you understand?
I thought we talking about lessons from rhetorical questions in general, and I believe that I proved my point concerning rhetorical questions' ability to stimulate question asking. If you want an explaination on the tree falling in a forest, I will have to think about what that question means to me for a while before I can give you an answer.