Every single year, without fail, I participate in a little winter ritual. I arrive at some snowy place, eager to strap on snowshoes or nordic skis and tromp or glide out to a charming hut or a beautiful overlook, or hopefully both. First thing I do is break out the disposable hand warmers. I tear open the little packet, remove the teabag-looking miracles, slip them into my gloves, and then wonder sets in: How the hell do these thing work? Are they even safe? If I puncture one in my glove, will my fingers be dissolved into some horrible chemical pudding?

Then my hands warm, and I ooh and aah at the wonders of science and I immediately forget all about those questions and carry on trudging through the snow to my hopefully picturesque destination.