They know how to use the catdoor after dad spend a weekend teaching them by example (the example was the dog) who was still a puppy that fitted through the opening.

For other doors they think they need their personal door opener: me. If you scream long enough, mom will eventually get up and open the door next to the cat door. It's always very funny to see Rudie with his mouth in 'yell' position because he starts silently and increases the decibel.

The livingroom door is always for so much that a cat fits through. If a door upstairs is closed, the cats start ripping away the carpet (not funny anymore!)