First, drill a hole in the two bottles corks.
Then, fill one of the bottles with water to about three quarters and join the two plugs per cylinder.
To unite the two bottles it can be used a tape.
It is very important the union between the bottles.
When the water on the bottles is empty is upper, it seems that the water does not fall easily to the bottle bottom, but if we give a circular motion to the bottle top and creates a swirling, water falls easily.
By placing the bottles on top, water doesn’t fall to the bottom because of the compressed air trapped in the bottle which is down. It can pass through it, as there is no room for it.
When the eddy is generated by moving the top bottle, it is communicated the air in both bottles and the water in the top bottle falls easily into the bottom bottle.