Some believe plastic-and-foam skull caps make skiers feel invincible, and therefore "provoke" injuries, but Flake's team of researchers couldn't prove helmets guilty. Sure, it's comforting to know that quality helmets - those carrying a CE, ASTM, or Snell RS-98 certification - undergo impact tests in which an 11-pound, helmet- swaddled, magnesium dummy head (your dome weighs between eight and 11 pounds) is dropped at 13 miles per hour onto a steel anvil. And it's consoling to hear that if your $100 egg crate can't handle 300 G's of acceleration, it gets shipped to Dumpster City. But a helmet won't necessarily save your noggin if you slam it into an immovable object at 30 miles per hour. Think of it the way an N.R.A. member would: Helmets don't crack skulls; people (meaning, you) do.