Ancient Egyptians believed that when the pharaoh died, he became Osiris (king of the dead) and his spirit would remain with his body.
They also believed that if the corpse did not have proper care, the dead pharaoh would not be able to assume the role of Osiris.
To prevent such a thing from happening, the ancient Egyptians would preserve the body of each dead pharaoh in a process called mummification. Everything the king would need in his afterlife was buried together with him in his grave—vessels made of clay, stone, and gold, furniture, food, etc.
To safeguard the pharaoh's dead body, spirit and personal belongings, Egyptians built massive tombs. Some of the tombs were in the form of pyramids. Before the pyramids, the Egyptians built mastabas - rectangular flat-roofed tombs.
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History
The Pyramid Of Khafre
How Pyramids Are Built