Pyramids Mystery Solved!

Ancient Egyptians had to pull massive statues and pyramid stones weighing 2.5 tons on large sleds across the desert -- without any modern mechanical device. Now, new research shows how adding a small amount of water to sand significantly reduces the sliding friction -- a clever trick that allowed the Egyptians to cut the number of workers needed by half.

To make a good sandcastle, you don’t use dry sand. By adding water, the grains stick to each other, and your castle holds its shape. Same thing with sand transportation: Adding water reduces the sliding friction of any object moving over the sand. With the right amount of dampness, water droplets bind the sand grains together.
An international team led by Daniel Bonn from the University of Amsterdam tested the sliding friction of dry and wet sand by pulling a weighted sled across the surface in a tray. With dry sand, a heap would form in front of the sled, hindering its movement. And as they added water, both the force needed to pull the sled and the amount of friction decreased.