For a while, I've been having the occasional dull ache in the lower back. I figured that it was sleeping in my customary discus-thrower pose on the new, Korean mattress my wife had bought a few months back. Being new, and in particular being Korean (although cunningly named 'Lady Americana' to give it that so-important New Jersey cultural cachet), it is approximately as hard as a slab of granite. Not that soft, dissolute western granite, either. Good, hard, Korean sleeping-granite, ripped from the very earth in the mattress mines of Kangwon-do