资料题：What is this passage mainly about（　）。

Some of the most interesting buildings in the world are the pyramids.The pyramids stand huge and silent, and in modern days, people visit them and wonder, ''Who built them? Why ? When? What is inside? How did they build it?"Thousands of years ago, some king of Egypt built the pyramids.They built them as tombs.The king thought the pyramids would help them find life after death.They also wanted the world to remember them as important people.Some pyramids were for queens, but they are less interesting because they are not as big as those of the kings!Today we know that the oldest pyramid is the pyramid near Sahara in Egypt.It is about 5,000 years old.There are many pyramids along the Nile River.The largest is the pyramid of Khufu.It is made of more than 2, 300, 000 huge stones.Most of the stones are taller than a person.It is about 144 meters high.Inside the pyramid are the burial rooms for the king and queen and the long ways to these rooms.The eat of the pyramid is made of solid stones.Workers usually built the pyramids when they had little or no work to do on their farms.To build the pyramid of Khufu, 100,000 men worked for twenty years.We know there are wonderful treasures on the pyramids.Robbers have dug into some of the pyramids and taken away some of these treasures.However, today some of the treasures are in museums.How did the people of ancient days build the pyramids? How did they carry and lift up the huge stones? Each stone fitted in with one another so well though they didn't have our modem machines at all! The ancient art work of Egypt gives us the idea of a miracle.Scientists have studied the pyramids, but nobody can say just how they built it.