Since first posting the entry below, I've had some excellent advice from readers on dealing with the whys. Flash programmer Minty writes from Sydney that he favours the dada-Daddy approach to circuit-breaking: "When it's obviously getting circular, my answer becomes 'Chocolate cow.' An example:
Him: "What's that?"
Me: "A rubbish bin"
Him: "What's that?"
Me: "A grey rubbish bin"
Him: "What's that?"
Me: "A chocolate cow"
Him: "Noooo! It's a grey rubbish bin."
And thus endeth the conversation, with a child happy in the knowledge that he's corrected my inability to recognise a rubbish bin."
And Brent Jackson gets to the bottom of the whole thing by re-interpreting the question in a very intelligent way:
Just thought I'd share a small breakthrough we had with our toddling daughter regarding the cascading "But why?" questions. She was repeating the question because we weren't answering what she was asking. Once we figured out that her "Why?" was not "Why is it so?", but actually "Why do you think that?", or, less ambiguously "How do you know that?".
If my daughter had used your example, "Why is that man grumpy?", our response was along the lines of "I could tell he was grumpy because he tooted and raised a finger to me when I changed lanes."
Smart, that, and quite possibly definitive. I'll give it a go and let you know.