How ancient Egyptians constructed the pyramids is still somewhat of an archeological mystery. Everything from cranes and ramps to oil-slicked slipways to aliens (naturally) have been put forward as possible mechanisms. A group of Dutch physicists has a new hypothesis on how ancient Egyptians managed to drag the colossal stones necessary to build pyramids across the desert. The answer: wet sand.
In a study in the journal Physical Review Letters, researchers from the University of Amsterdam and FOM (the Foundation for Fundamental Research on Matter) recreated a laboratory version of the sledge on which workers hauled heavy stone, and tested how it fared in sand. They found that pulling the sledge across damp sand requires only half the force of hauling it in dry sand. Because the water droplets bind the grains of sand together, wet sand is twice as stiff as dry sand, and doesn't pile up in front of the sledge as it moves along. (A good tip for sandcastle construction, too.)
The researchers bolstered their theory with a wall painting from around 1880 B.C. found in the tomb of a 12th-dynasty administrator named Djehutihotep, which shows what looks to be a worker pouring water in front of a sledge carrying a large statue.