<https://www.prepperforums.net/threads/soda-beer-can-fire-starter.6348/>
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"This is an item I had in a survival video I made in 1994\nYou're on a survival setup and you come across a beer can---DON\"T SMASH IT.  If it has a concave bottom you can use it for fire starting. The bottom is like a magnifying glass in reverse. Use a finger to go round and round until it gets shinny. Then use your Tee shirt to shine it to a higher gloss. Line up the bottom facing the sun and put some tinder at the focal point. With dry and proper tinder you have a fire going."
<https://www.prepperforums.net/threads/soda-beer-can-fire-starter.6348/>