Grant Thompson - the King of Random - teaches me how to start a fire with a Sandwich bag. And we tried to melt glass in my backyard: goo.gl/zb0uE0
Check out his channel: ve42.co/KoR
The intensity of sunlight on Earth is about 1300 Watts per square meter. When you focus the sun's rays using a magnifying glass (or in this case sphere of water) you can increase the intensity roughly ten thousand fold. This increases the temperature of wood to its autoignition point starting the reaction with oxygen in the atmosphere. By protecting the hot embers and adding more energy and fuel, you can get these hot coals to start a roaring fire.
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