A chain fountain is the name given to the counterintuitive phenomenon where a long bead chain
appears to defy gravity by first leaping out of its container before falling to the ground.
This became known as the Mould effect, after a British science presenter, Steve Mould, who made the experiment famous with a video that went viral on YouTube.
Apparently, he discovered the chain fountain phenomenon while looking for a way to explain at the molecular level the capability of viscoelastic fluid to pour itself, the so-called open siphon effect.
In this funny TED talk, Steve Mould recounts his discovery and investigation into this entertaining and counterintuitive phenomenon.
The physics behind the chain fountain is non-trivial.
At first sight one could be tempted to explain the phenomenon by observing that the falling chain has downward momentum, causing an upward momentum in beads leaving the container. This, in turn, makes them leap before gravity can slowly reverse their momentum. However, if inertia causes the flowing fountain, the chain would be stationary at the top of the curve, while this is not the case.
It has been later proposed that the fountain is not driven by inertia, momentum, or gravity but rather by an anomalous push force exerted by the container on the link of the chain about to come into motion.
The authors of the paper also posted a video on the Royal Society Youtube channel where the educational value of the demonstration and the analysis is highlighted.
Since it was first brought to widespread attention by Mould, many explanations of the chain fountain have been put forward and several papers have been published, even recently (http://aapt.scitation.org/doi/abs/10.1119/1.4980071, https://journals.aps.org/pre/abstract/10.1103/PhysRevE.89.053201, https://arxiv.org/abs/1612.09319).
However, a complete description of the mechanisms at play appears to be still lacking.