You would need one brick – the last one. The “box” here is that you are “captured” by the concept of a whole wall, which eventually can’t lead to any logical answer – how big is the wall? How big are the bricks?…
Instead, it is the word “complete” that facilitates an “out of the box” thiniking process, because in all cases, it is the last brick only that would make any wall complete!
The river is frozen. The “box” here is about thinking of a river in only one way: As a flowing river!
They are triplets. The “box’ here is that you limited your explanation to a “fact” that is however not stated. There is no mention that the woman gave birth to two babies ONLY!
The fastest way is to go to the airport and travel by plane. The “box” here is the car, but there is nowhere made clear, that these persons should travel by car, just because they find themselves in one…!
He fell out of a ground floor window. There is nothing stating or implying that the man was in a high floor. The box “here” – restricting your brain – was that you have been lured into thinking of a fall from a great height, since the building is a 30-stories building as described.