Dragging a heavy stone across desert sand sounds like a job for brute force. A painting in an Egyptian tomb suggests the real trick was a bucket of water.
In the tomb of an Egyptian official called Djehutihotep, a wall painting from nearly 4,000 years ago shows 172 men hauling a colossal statue on a sledge. Standing at the front of the sledge, one man pours liquid onto the ground in front of it. For a long time this looked like a ritual, or maybe a bit of cooling for the ropes.
In 2014 a team of physicists led by Daniel Bonn at the University of Amsterdam tested another idea: the water was there to make the sand easier to cross. They published the results in Physical Review Letters, in a paper called "Sliding Friction on Wet and Dry Sand".
Pull a sledge across dry sand and the front edge digs in. The sand there is loose, so it does not carry the weight, and the runners sink a little. The sledge then pushes a small heap of grains ahead of it, like a bow wave, and the crew has to keep shoving that heap forward as well as the load.
The researchers dragged a weighted sledge over trays of sand in the lab. When the sand was dry, the sledge built up a heap and the pull needed was large. When the sand was damp, the heap did not form. Water makes tiny liquid bridges between neighbouring grains, and those bridges glue the surface together. The sand turns stiffer and the sledge rides on top of it instead of ploughing through it.
The effect is not "more water, less friction". Reports on the study put the useful amount at only a few percent of the sand's volume, around 2% to 5% depending on the sand. Too little and the bridges are few and weak. Too much and the water fills the gaps between grains, the glue turns into slurry, and the stiffness drops back toward that of dry sand.
At the sweet spot the sand was about twice as stiff as dry sand, and the force needed to pull the sledge was roughly halved. In other words, a crew that could move a load with dry sand underfoot needed only about half as many people once the sand was wet in the right way.
This is a simplified model, not the lab data: a stiffness curve with the same shape as the one the study describes. Slide the water content and watch the heap in front of the sledge. Then find the amount of water that makes the crew's job easiest.
Illustrative model. The real optimum depends on the sand, and the numbers in the study are measured forces on a lab sledge, not head counts.
The experiment shows that wetting sand cuts the force needed to drag a heavy sledge across it, and that the painting fits the method. It does not prove that the builders of the Great Pyramid used it. The painting is some six centuries younger than that pyramid, and no text says why the man is pouring. It is a physically sensible explanation for a puzzling scene, not a signed confession.
It is still a lovely example of a rule that engineers keep rediscovering: a granular material can behave very differently once a little liquid gets in. It is the same reason a sandcastle needs damp sand, and why it collapses in a heap when the tide arrives.