The water bottle flip challenge became very popular very quick but it is not just luck in getting the bottle to land upright. When the bottle is moving through the air during its flip, the water begins to climb the sides of it. This causes an increase in it's moment of inertia which in return, decreases its rotational speed. This is very similar to a diver who extends his arms and legs in order to slow his spin or an ice skater. The amount of water inside of the bottle also plays a major role in the success of it landing upright. If the bottle is full, the water would be unable to climb the sides and it would give it a high center of mass. That means when it lands, it would just topple over. After experiments to determine the best height, it seems that filling it just below a third of the way produced the best results.