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Canned food is an excellent resource for survival scenarios, as it's compact, flavorful, and has a long shelf life. However, if you have canned food without a can opener, you may be stuck with a difficult decision. Sure, you could try hacking the can open with a knife, but you run the risk of cutting yourself or leaving metal fragments in the food. Fortunately, there's a better way.
Understanding how a can is sealed will give you a hint at how it can be opened. The tops of most cans are sealed at the factory by folding over or crimping the metal rim. If you can grind off this crimped rim, you'll be able to pry off the top of the can with ease. Some elbow grease and a flat concrete surface is perfect for this task. Here's an example:
In this example with a can of tuna, you'll notice that when the edge begins to separate, moisture starts leaking out. This can be a good indicator of when you've ground enough material off the can—if the concrete starts getting wet, you're almost done.
The above video uses a knife to pry open the can, but you could easily use your keys, a pen, or even a thin stone. Obviously, all of this can be avoided by carrying a small can P-38 can opener or multi-tool, but it's still a good backup plan in case you're left empty-handed.