The (not so simple!) chain fountain
  Given a sufficiently long bead chain in a cup, if we pull the end of the chain over the rim of the cup, the chain tends to continuously flow out of the cup, under gravity, in a common siphon process. Surprisingly enough, under certain conditions, the chain forms a fountain in the air! This became known as the Mould effect, after Steve Mould who discovered this phenomenon and made this experiment famous on YouTube, in a video that went viral. The reason for the emergence of this fountain remains unclear. This effect was shown to be due to an anomalous reaction force from the top of the pile of beads, a possible origin for this force was proposed in the same paper. Here, we describe some experiments that give a contribution towards the clarification of the origin of this force, and show that the explanation goes far beyond the one proposed before. 
Rog\'erio Martins
physics.class-ph