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.
From Veritasium‘s Derek Muller and King of Random‘s Grant Thompson, learn how to start a campfire with a sandwich bag full of water. Since these adults couldn’t contain the fire in a firesafe container, this video is also an excellent conversation starter for fire prevention and safety, as well as for using matches (and plastic bags of water) as tools. As always, be safe, smart, and don’t conduct fire experiments without an adult present.
Related vids include Candle Chemistry, The Goat Brigade, The Fire Lab and the Mysterious Science of Fire, What is Fire? and How do firefighters combat and control wildfires? Plus, from Primitive Technology: Making a cord drill & pump drill from sticks & rocks.