Genesis 7:2 Of every clean beast thou shalt take to thee by sevens, the male and his female: and of beasts that are not clean by two, the male and his female. 

Very early in the Bible, Noah knew about clean and unclean animals. He knew about sacrificing certain animals during worship time. 

God gave explicit instructions about the clean and unclean animals in Leviticus 11. That will be hundreds of years after this chapter, but Noah already knew about clean and unclean animals. 

So much of what we find in the Bible is not showing all the details. How did Abel know what offering God expected and demanded? Why would God get angry with Cain if he did the best he knew how and could?

In the very beginning, God taught Adam and Eve how to worship Him. The written revelation will come hundreds of years later. For example, as Noah prepares to build an ark to save the world, he is told to take on a certain number of clean animals compared to others. 

What were clean animals? What were unclean animals? It is fair to assume that Adam and Even knew and taught their children. Noah just knew! Noah had parents who had parents that gave their lives to teaching their children the truths of God. 

Noah knew what to do with the sacrifice. He knew about which animals God commanded for the sacrifice. Noah had parents that taught him.

So the question is, are you teaching your children? Think about this. How else could Noah know what to do? The responsibility to teach our children is an exceptional and weighty one.