How to open a can without a can opener

First, you're going to need a metal object. I would imagine that a very sharp piece of hardwood might work if you're really persistent.
Now, hit the side of the cylinder toward the top over and over again, until there's a dent in the side.
Continue by hitting that dent again and again until you're able to pierce the point you've been hitting.
If it's a can of broth or juice, your work is done. Enjoy.
If it's a can of something else-- Ravioli, for instance, the game gets trickier. If you have some silverware lying around, take the handle and jam it into the hole you made in the can.
Now force the end sticking out of the can one way until you strip away the side of the cylinder that's attached to the top.
Now, force it in the other direction, effectively giving yourself about 200 degrees around that cylinder to work with. At this point, you should be able to pry the top back enough to get what you need.
You've now succeeded in a feat few have accomplished since 1930 without the use of a can opener, ginsu knives, a group of hobos heating the can over a fire, or your friend from college who can open cans with his face.
Enjoy the foodstuffs within, for you truly are the Prometheus of our day.