I keep a credit-card-sized fresnel lens in most of my kits. As hikermor points out when you most need a fire it will least likely to work but I have used them to remove splinters, read small print on medicine bottles, etc. A fresnel is great since it's light, flat, flexible and very durable.
Failing eyesight is the main reason I keep one in my kits too, Phaedrus. They are fun for firecraft practice but I wouldn't want to depend on that method in an emergency.
Sidenote: a Fresnel lens and a pencil sharpener are a fun way to introduce kiddos to fire-making.
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