Two bottles made of plastic 1,5 l.
First, we make a 1 cm hole at the tops of the bottles. Then, we fill one of the bottles with 75% of water and join the bottles by means of the tops. To join the two bottles we use the insulating tape. This joint is very important, so do not be stingy with the insulating tape.
When the bottles with water is on top of the other, the water does not fall easily to the empty bottle, but if we make a circular movement with the upper bottles, it generates a whirlpool, and the water falls easily.
When we place the bottles full of water on the top, the water does not fall because of the low compressibility of the air, which takes up the lower bottle and does not let any space for the water to fall.
By the time, the whirlpool is generated by the air of the two bottles that passes from one to the other and the water of the upper bottle falls easily.
By Mercedes and Fátima