  In this article we show why flying and rotating beer mats, CDs, or other flat
disks will eventually flip in the air and end up flying with backspin, thus,
making them unusable as frisbees. The crucial effect responsible for the
flipping is found to be the lift attacking not in the center of mass but
slightly offset to the forward edge. This induces a torque leading to a
precession towards backspin orientation. An effective theory is developed
providing an approximate solution for the disk's trajectory with a minimal set
of parameters. Our theoretical results are confronted with experimental results
obtained using a beer mat shooting apparatus and a high speed camera. Very good
agreement is found.
