Usually, there is always a knife or some tool but in some extreme situations (war etc.) it could happen that you have to use this or some similar way for can opening.
Step 1: Watch the Video!
Find a flat rock or a piece of concrete and just rub a can against it. Look for one with a rough surface. A smooth rock won't create enough friction to puncture the lid of the can. Position the can upside down against the rock. Positioning it upside down will allow you to break the seal, which is located on the top of the can. Rub the can back and forth over the rock. Keep going until you see moisture appear on the rock or on the lid of the can. If you have a tool of some sort, you can pry it open. Otherwise, you’ll have to keep grinding until the lid comes off easily, so you can remove it with your fingers. If you don’t want to consume metal filings or concrete dust, just wipe the edge of the can before opening. It should remove most of the loose material.