You don't always have a can opener, but you probably do have a spoon. If you need to open a can and a spoon is all you have, you can still open that can.
As demonstrated in the video above, just push down with the tip of the spoon against the inner edge of the can and start rubbing. In a short while, you'll penetrate the lid and you can carve around the can with the spoon's edge. It's certainly not as elegant as a can opener, but it will do in a pinch.