The unification of Upper and Lower Egypt by King Menes occurred between 3100 BCE and 2900 BCE. This marked the beginning of the first Egyptian dynasty, and King Menes became the first pharaoh, or supreme ruler. The pharaohs who followed Menes strengthened their rule over the kingdom and increased their personal prestige. They built elaborate pyramids for their tombs, beginning with the step pyramid of the Pharaoh Djoser and including the Pharaoh Khufu's Great Pyramid at Giza. About 100 pyramids still exist today. They are joined by dozens of large and small obelisks, which are pointed stone pillars built to commemorate the great deeds of the pharaohs, worship the sun god Ra, and provide magical protection and stability for Egypt's tombs, temples, and kingdom. Egyptians are also famous for our hieroglyphics, a system of picture writing that developed sometime around 3300-3200 BCE and contained between 700 and 800 glyphs, or picture symbols that represented objects, ideas, or sounds. Along with hieroglyphics, they also used the simpler hieratic and demotic scripts for everyday purposes. Finally, by studying the stars and the moon, as well as the cycle of the Nile's floods, Egyptians created a very accurate calendar with 12 months and 365 days.