Walls are a natural enemy to pets. Walls stop pets from getting out to socialize with other animals, and also stop pets from getting back in after strolling. Walls stand between freedom and comfort, and pets have to overcome them several times per day. Which of course raises the question how pets can alert humans to let them back in once they return.
Limit #6 Getting back in
Cats are limited, but quite smart. Lacking the understanding of the door bell, they have to show up in front of the window to get in if all doors are locked. The most reasonable window to check can be easily spotted by the marks these pets have left on the wall over the past decades.