Bodies can have very weird trajectories when tossed into the air. The amazing motion of a water
bottle flip is qualitatively described as related to the position of the centre of mass (CM) with
respect to the body’s frame of reference. Although this is rather easy to guess from simple
observations, it is not so obvious to verify quantitatively with the naked eye. In this work, we use
video analysis and a computational model to show that the position of the CM changes in the frame of
reference of the bottle, while it describes a parabolic trajectory in the frame of reference of the
laboratory. We also show that the behaviour of the CM also explains the weird motion of the bottle.
It can be observed graphically and discussed in the classroom with students, as it only needs some
critical thinking and practically no math.