First, the Lord tells him:

“You are to bring into the ark two of all living creatures, male and female, to keep them alive with you. Two of every kind of bird, of every kind of animal and of every kind of creature that moves along the ground will come to you to be kept alive.” (Gen 6:19-20)

Noah does this. But then, out of the blue, God hands down a different set of orders:

“Take with you seven of every kind of clean animal, a male and its mate, and two of every kind of unclean animal, a male and its mate, and also seven of every kind of bird, male and female” (Gen 7:2-3)

Noah must have lifted his eyebrows. Again, he obeys – after all, the Lord is trying to save his life and that of his family. But then, probably to Noah’s confusion, the animals begin to show up in pairs only:

“Pairs of clean and unclean animals, of birds and of all creatures that move along the ground, male and female, came to Noah and entered the ark, as God had commanded Noah. (…) Pairs of all creatures that have the breath of life in them came to Noah and entered the ark. The animals going in were male and female of every living thing, as God had commanded Noah.” (Gen 7:8-9, 15-16)

Of course it didn’t really happen that way. In fact, most bible researchers agree that Noah’s tale is written not by one, but by two authors, at different times.

Author #1 came up with the version where the animals came in pairs. But author #2 disagreed, and felt it was not right that author #1 treated the clean animals equally to the cockroaches, the snakes and the worms – the unclean animals. So he introduced God’s new command: “Bring in more clean animals than unclean animals!”

Unfortunately, the second author forgot to modify the older version of the story.

The two authors disagree over more things. For example, we see Noah and his family entering the ark two times, in Genesis 7:7 and 7:13. Author #2 says the world flooded because it rained, while author #1 writes that the water came from below the earth too.

The two authors even disagree over how long the flooding took. Author #1 mentions that it rained for forty days and nights (Gen 7:4,17), author #2 says that it rained “for a hundred and fifty days” (Gen 7:24)

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Many critics have pointed out that the story with the animals doesn't make sense. Where did Noah stash the 2 million species of animals biologists know of? How did the animals cross the oceans to end up in places like Australia and the Americas after the flood?