This is a very useful skill, because often there's no can opener to be found.
In fact, there were fifty years when cans HAD to be opened without a can opener.
"The first tin cans, invented in 1810, were heavy-weight containers that required ingenuity to open, using knives, chisels or even rocks. Not until cans started using thinner metal about 50 years later were any dedicated openers developed."
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Step 1: Advanced Technique: Open Can With Ballpoint Pen
From my Yucatan Proa trip log:
A Bic ballpoint with the hard clear body as seen here is good for this.
Some pens are made of soft plastic that isn't as good. This old model is great. It survives salt water and all kinds of abuse. The ink doesn't run when it gets wet. I liked it better than an "astronaut" pen that failed on the same trip.
Open Can with Pen:
1: Using a corner of the pen's blunt end, score the can across top. This makes a big "X" in the top of the can.
2: Stab a hole in the middle of the top with ballpoint. This is easier than it sounds.
Now there's a hole in the middle of the X. A hole with four corners.
3: Enlarge the rips in the four corners. Push each corner along with the handle of the pen til it reaches the edge. Pry up each flap.
4: Eat contents with two sticks as chopsticks. Plan uses for the interesting metal thing you have left.