- They come folded up to fit in the palm of your hand, and open up to cover your body, and more. It's shiny, and made from a Mylar plastic, coated with aluminum, that reflects up to 80% of your body heat. It's shine also is great at signalling.
- A larger version is a big bigger when folded, so takes up more space in your supplies. It isn't as reflective or as shiny, but it is more durable. It's made out of a tougher woven material, and has grommets in the corners to allow for tying down. These are smaller than a tarp but can cover 2 people easily. They can also be used as a ground cover, windbreak or body wrap. They also keep rain and snow off in a pinch. These come in neon-type colors which will help signaling for rescue.
Use these to insulate you against cold OR to reflect heat from, say, a desert, away from you. You could also use them to line a solar oven, disinfect water, or reflect the sun when hung in your windows.
I've found these at Wal-Mart for .99 cents each. I grab all I can find when they are in stock; usually, that space is empty!
In my opinion, you can't have too many of these.