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.
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“I'd rather have questions that cannot be answered than answers that can't be questioned.” —Richard Feynman