Which difficulty level should I choose for my dog’s toys and puzzles?

Think about crossword puzzles for us humans, we need to start easier to understand the concept, and step by step advance to more difficult versions. If we start too difficult, we will be frustrated and finally give up. A dog who doesn´t understand how a puzzle works, can by instinct start chewing their way into the treats, instead of working for them, which is completely normal for a dog if we don´t teach them the playing rules, and how the puzzle works.