It is common knowledge that when a stone is thrown into a pond, a jet of water shoots upwards. High-speed video imaging and even snapshots taken with short exposure times reveal that the jet breaks up into droplet patterns. Such patterns have inspired artists such as Andrew Davidhazy. What is both remarkable and intriguing is that very similar arrangements are observed in the streams that emerge when a stone is dropped into loose, fine sand. The common view, which we have also adhered to, about the origin of this striking, liquid-like behaviour of streams of sand or other granular materials has been that the inelastic nature of collisions between the grains causes them to cluster together into larger structures. But on page 1110 of this issue, Royer and colleagues show that this view is wrong, and that forces hitherto believed to be too small to cause the clustering are at work.
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