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5,000 years ago, without computers and machines, how did the ancient Egyptians built hundreds of massive hundreds pyramids in the African desert?  Did they use half a million workers to build just one pyramid over 20 years?   Now physicists at the University of Amsterdam have found the answer.  They investigated the forces needed to pull weighty objects on a giant sled over desert sand, and discovered that dampening the sand in front of the primitive device reduces friction on the sled, making it easier to operate.  Adding water to the sand, however, increased its stiffness, and the sleds were able to glide more easily across the surface. This is because droplets of water create bridges between the grains of sand, which helps them stick together, the scientists said. It is also the same reason why using wet sand to build a sandcastle is easier than using dry sand. But, there is a delicate balance, the researchers found. "If you use dry sand, it won't wo...