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The levels have different degrees of difficulty. Of course there is no real objective standard of how difficult a level can be. In the end it all depends on the player, his or her skill and preferences. One would find levels where it mainly depends on skill and quickness, easier than levels where you have to puzzle a lot to find the solution. For another player it would be the other way around.
Some levels also seem unsolvable if you don't find or notice the way to the solution, or with which tricks a seemly complicated puzzle is solved. In many levels you can get some clues after a certain amount of failed attempts are shown.
Here are the different degrees of difficulty:
Should this level not be easy to accomplish and you're getting more frustrated and feel like throwing the computer out of the window, there is always the possibility of getting a cheat code of this level.