Now, I'd never heard of these things before, but they're simple closed cloth tubes with polymer crystals inside that work on a simple scientific principle: evaporation. The crystals absorb moisture and swell, and slowly release moisture which cools you down. It sounds sort of silly, and they look sort of silly as well (like a damp, fabric covered sausage), but it was hot, so I bought one.
I'll be dipped if the things don't actually work like a dream.
Though the neck coolers will dry out eventually (for winter storage), it takes more time than I had on the coast, and I didn't want to have to try to explain what it was to the TSA folks, so I left mine with my sister, but as soon as I got home, I ordered some of the polymer crystals so I could make more. (Note: you might be able to find these retail at a craft or garden store, but there were none to be found locally in my neck of the woods), and I found instructions for sewing the coolers online here. (my only adaptation was to use just 1 1/2 tsp of the crystals, which was plenty for the tube).
I wanted to see how much the crystals would actually expand before putting them in the fabric tube, so I started with 1 1/2 tsp (1/2 tbsp) and a little half-cup container.
I added water a tbsp at a time, and within 5 minutes, the crystals had out-expanded the container.
15 minutes later, they finally stopped growing. That's a good cup of mooshy stuff there, all from 1 1/2 tsp of crystals. The fabric tube is ready for the crystals (I didn't try to add the expanded ones- I used dry).
After 3 minutes in the sink, it had already starting to plump up.
And within 10 minutes, it was fully expanded and ready to wear (the ends get wet the first time, but they dry, and usually don't drip later on).
Voila!- instant temperature control!
When the cooler warms up and the fabric dries a bit, just wet it again, and you're good to go. Store it in the fridge (not the freezer) for a really cool experience. The coolers are supposed to last a couple of years, but they use so little fabric, and take so few crystals (my little packet will make at least 10 coolers), that even if they only last one season, they're still worth the time and money. I'm going to make a few for gifts.
P.S. Yes, it does look a goofy, but it feels so good that I don't care.