There is a bird in a cage. It can reach its head out of the bars far enough to peck away at the wood of the door. After year’s of pecking it has finally broken through the wood and is free to escape. As the bird leaves it’s cage it realizes that it no longer remembers how to fly and falls to its death.
There is another cage with a mouse in it. One side of the cage has no bars, just a ledge. The mouse can’t see how far below the ground is and is afraid to jump. After year’s of living in fear of making the leap of faith the mouse dies in its cage.