I remember when I was about 10 I asked my teacher this question. He had always been a good teacher, encouraging open-minded debate, so I was quite shocked when

I remember when I was about 10 I asked my teacher this question. He had always been a good teacher, encouraging open-minded debate, so I was quite shocked when he told me not to ask stupid questions. (In retrospect, he'd probably just had his fill of me asking unanswerable questions that day.) Glad to finally get a detailed answer, 39 years later!