You ever look at a hangnail, or some other noxious infected bodily blight, and wonder why it's green? Why is infection goo green? Has it got something to do with copper? Copper burns and oxidises green. Yes, that is why the roof of the provincial parliament in Toronto is green; because the roof is partly copper. I always liked that section we did in grade nine science these many moons past, where we sat in the dark and watched the teacher burn various metals and such. I still can't remember, though, whether it was strontium or barium that burned purple.