How were the Egyptian pyramids built?

Pyramid building evolved overtime. The first Pharoahs were buried in mud brick pit tombs known as Mastabas (the Arabic word for step). Pyramid building began during the third dynasty 2686-2613 B.C.E. The Pharoah Zoser 2686-2649 B.C.E. was the first pyramid builder, his architect Imhotep began stacking mastabas thus creating the Step Pyramid at Saqqara. Tomb building became more elaborate during the fourth dynasty 2613-2589 B.C.E. beginning with the Pharoah Sneferu. Most archaeologists agree that his first attempt collasped, the second known as the 'Bent Pyramid' was built on unstable ground, and the third known as the "Red Pyramid' was the first true pyramid. It was Sneferu's son Khufu who built the 'Great Pyramid' at Giza. It was built on 13.5 acres, has 2.5 million blocks each weighing 2.5 tons each, and it stands 480 feet high. Khufu's son and grandson also constructed their own pyramids on the Giza plateau, however both are smaller than Khufu's. Since no evidence has been found to confirm how these massive tombs were built, most archaeologists believe a system of ramps were constructed around the pyramid which allowed the workers to pull the stones up on. They also believe that the pyramids were built with free labor not slaves. Archaeologists do not agree with Heredotus' 100,000 men 20 year theory. They estimate construction at 67 years with approximately 20,000 workers.