Let’s say we have two balloons of the same size, but one filled with twice as much air as the other one. We then connect both via a flexible hose and let the air flow through. What will happen? Will the pressure in the one that has the most air cause the other one to fill up so that their size evens out after a while? The following video has the answer, which might surprise you.
Another good explanation of the phenomenon can also be found here.