You can open a can without a can opener if you have enough elbow grease. A rock, concrete, or sandpaper will do.
Simply invert the can and vigorously rub the top along the rough surface. This will erode the soft metal on top of the can, eventually causing the seal to separate.
You can periodically press the can to determine if it has been worn down sufficiently, or you can wait until it begins to leak.
At this point, you can likely remove the top by squeezing it like Popeye grabbing spinach, or by inserting a knife or other flat object into the seam and lifting off the top.
Simply pay attention. If your concentration wanders when rubbing the can, the deal may be completely worn away and your food may end up all over the place.
Also, watch out for metal shavings—just like sanding wood, this process will produce some metal "dust" that you'll want to avoid getting into your meal.
If you have a flathead screwdriver, tap the tip of the screwdriver along the edge of the can until you can put it under the newly formed lip and peel the top up.
The spoon method involves grinding the spoon along the can's inner edge until it deforms, then inserting the tip and rocking it up and down to slice through the seal.