What about the plastic pouch inside the cereal box? I swear mine hacks it open with a dull rusty machete making it completely impossible to reseal or to pour the contents into a bowl without showering the kitchen floor with Frosted Flakes.
That was an easy fix - Tupperware cereal bins. That way the only thing they can do to screw it up is leave the lid open so the cereal gets stale. And since I don't eat cereal, I didn't care.
My bugaboo was the toothpaste. They'd leave the cap off so there's this paste crust you have to chisel through in order to squirt it on your brush.