Not sure when this became a thing, but it feels relevant and might be useful to someone someday.

a formerly homeless friend once showed me that taking a brick and grinding it on top of the can, will open it without a knife. the 'ridge' of the can, the metal circle that runs along the diameter at either the top or bottom, is a metal 'lid' that's folded or pinched shut onto the other piece of metal, the 'cup'. (single quotes around terms i picked and might not be official jargon)

in just a few minutes, the brick ground the metal off the ridge, seperating the lid from the cup, which easily popped out. technically you don't really need any tool, just some relatively flat concrete or a rough flat rock. or even low grit sandpaper.

i don't recommend tossing out the can opener though, there is a chance of metal shavings falling in if you aren't careful. still might be useful in an emergency.

Many Swiss Army knives have a can opener. You hook a part of it under the can rim and it acts as a lever for the small knife blade above it. You simply work your way around the can, cutting the lid a bit at a time. I've done it many, many times. It's safe and easy.

https://www.google.com/search?q=swiss%20army%20knife%20can%20opener&ie=utf-8&oe=utf-8&client=firefox-b-m

I think hammer and chisel was the common way.