Why Egyptians poured water on sand: the wet-sand trick for dragging giant stones

A wall painting in an Egyptian tomb shows 172 men hauling a colossal statue on a sledge, while one man pours water onto the sand in front of it. In 2014, physicists led by Daniel Bonn tested the idea and found that damp sand is about twice as stiff as dry sand, which roughly halves the pull needed. Slide the water content in a simplified model and find the Goldilocks dampness: too dry and the sledge ploughs a heap, too wet and the grip turns to slurry.