Lift the removable bottle to the top and let all the water run out of it. Lower the bottle to the bottom You see a fountain in the lower fixed bottle. When you raise the removable bottle, the water ran out of it. And it flowed in to the higher fixed bottle. When you pulled the removable bottle down, some water from the lower fixed bottle flowed into it. This decreased the pressure in the lower fixed bottle. The water was sucked from the higher fixed bottle. So the fountain was forme.