I don't know if possum yarn comes from those that the Dept of Conservation traps and disposes of, or if there are possum yarn farms (can you imagine the shearing sheds? Big guys hanging in harnesses wrestling possums to the floor and attacking them with miniature barbers' clippers and then sending them back out, naked and surprised, into the barn?), but either way, the reason it is so exceptionally warm is that possum fur, as is, is hollow. This is difficult for me to get my head around; it is obviously on some sort of nano-sized level with which my brain is not really comfortable, but the hollow fibre gives a couple of atoms' width of trapped air and keeps the possum nice and toasty.
The presque-finished sock on the needles in the first photo is turned over and hidden, cos it is a gift sock, but this is a sneak preview of the astonishingly lovely cable pattern I picked from Handknit Holidays by Melanie Falick. I love these socks like you wouldn't believe. I wish I could make them for every single person I know.