The invention of the tin can fully transformed people's ability to store food and beverages for long periods of time, but at the start those wrought iron cans were a true pain to open, and actually required a hammer and chisel! The can opener wasn't invented until 50 years later in 1858, by which time, cans were no longer made out of heavy wrought iron. And the new lids were pretty thin.
Even though you normally have access to a can opener, sometimes you get caught without one. Maybe you're out camping or staying in a vacation rental that somehow doesn't have that implement. No need to worry; there are plenty of ways to break into that can. Just do yourself a favor and be very, very careful. Those edges are shockingly sharp! Here are some of the most highly rated ways to open a can without a can opener. (Whatever tool you use, always make sure it's cleaned and sanitized. No sense transferring bacteria or other goo into the food.)