What is the single best survival video or tool you have come across that has helped you or you think everyone should know?

For me a Fresnel lense has and was a epiphany. I found one in my dads office. He's a engineering and I just took it outside to be a pyro and it starts fires almost immediately where a regular magnifying lense just creates embers.

Its light and flat enough to fit in any pack and you can salvage Huge ones from old TV's and smaller ones from old screen projectors that they had in elementary school.