1) If humans are products of nature...
2) Our brains and the bodily tools we were given by nature to survive and advance are what make us human, so maybe they are products of nature...
3) So anything we invent [skyscrapers, aeroplanes, weapons of mass destruction, etc.] is a product of our tools, and maybe our inventions should too, be perceived as products of nature...
Given this, how is it possible to distinguish between something that's "man-made" and something that's "natural"? It's believed that man-made forces have terribly interfered with nature and caused irreversible environmental damage, but if we are part of nature, are we really interfering?
It's something that's always puzzled me!