Things fall down: Thanks to gravity, it’s one of the defining features of life—and physics—on Earth. So how to explain the chain fountain above, in which a 50-meter string of metal balls briefly flows up before plunging to the ground? Steve Mould, the BBC science presenter whose YouTube video recently drew attention to the weird phenomenon, has one explanation : The chain, which has to travel over the lip of the beaker before it can make its downward turn, is moving so quickly that it can’t instantly change direction. Instead, it continues traveling up for a bit, “chang[ing] direction slowly, over the course of a loop.” But when two Cambridge physicists tried to write an equation describing the chain fountain’s behavior, they discovered that Mould’s intuitive explanation was wrong. Math tells us that chains have no problem instantly turning even the sharpest corners, the physicists say, so they had to look elsewhere to explain the chain fountain’s surprising vertical leap. By working through the calculus, they discovered that the creation of the flowing loop depends not on inertia, momentum, or gravity, but rather on the upward force coming from the bunched-up end of the chain still in the beaker . In other words, as gravity pulls down, the chain itself pushes up—and it’s these opposing forces that create the fountain effect. Still, the physicists’ model—described online today in the Proceedings of the Royal Society A—doesn’t explain what causes the twists and waves at the top of the chain, leaving room for Mould’s explanation that they are the result of the chain hitting the lip of the beaker during its journey.