Cans nowadays are sealed with a roller. It bends the top of the can which creates a crimp (kinda like the way you seal empanadas, or any filled pasta). The friction cuts through the top, which makes it easier for you to open the can. Take note that the open top of the can will be sharp - so of course use it with caution, or else you will risk cutting yourself!
If you got lost in the woods or in a desert and you only have canned food in your bag, you will have to find a small boulder or rock that has a flat surface. If you're using a bigger can, you might need more time to get it to open. This technique is really cool and it's one of those survival tips that you really need to learn.
