How to Open a Can without a Can Opener

Apparently when the zombie apocalypse does arrive (any day now my friends, any day) and we all have to live off of canned foods, we won’t have to worry about using can openers. All you need is a piece of jagged concrete and some willpower and you can open any can, apparently. Might even work on the bark of a tree too, if the trees survive the zombies. Do zombies like trees?