Today on the First 40 Miles, Ounce for ounce, down is the warmest insulating material available—but where does it come from, how do synthetics match up, and did Neil Sedaka really write a song about his down puffy?  Then we’ll review a sleeping bag that not only harnesses the power of down, but also uses a unique closure system.  And if you’ve avoided down because of allergies, we have a simple trick that just may save you a box of Kleenex.