Simplify the following text for a Grade 5 native English-speaking person.
Original: The earliest Egyptian pyramids were step pyramids . During the Third dynasty of Egypt , the architect Imhotep built Egypt 's first step pyramid , the Pyramid of Djoser , by building a series of six successively smaller mastabas ( an earlier form of tomb structure ) , one a top of another .
Simplified: Step pyramids are a form of pyramid .