The trend of flipping a water bottle through the air to make it land upright again grew rapidly, and has some interesting physics behind it. The difficulty in the trick comes from the fact that when the bottle isn't full, it doesn't spin around the center. What happens is the center of mass goes toward the bottom as more of the liquid is drained, so to the average observer the bottle flips in a strange and unpredictable way. But the rotation of the bottle is predictible, because the axis of rotation is the center of mass of the bottle. If you know this, it makes it easier to predict the movement of the bottle. The easiest center of mass would be close to the bottom, at a point where the height if the liquid equals the diameter of the bottle. This seems to make the motion of the bottle very predictable, as it makes the bottle rotate around the bottom.