How were pyramids built

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The major wonders of the world are pyramids. There are many theories based on how that pyramids were built. Some say --- That Aliens have come once to the earth and they have made it for the business sake. Some say --- That Egyptians have built it. But I think 1 st one is correct because man can't build such a great one in just 2 years that too before the invention of the wheel What do you think