The distinction between clean and unclean animals came about by the use of sacrifices in worship, and not by what was permitted to be eaten and what was not permitted. The flesh of the animals was not permissible to be eaten by man prior to the flood. The clean and unclean designation were applied to the human diet at the receiving of the Mosaic Law, which also ended when the Law of Moses was abolished. (Acts 10:9-16; Ephesians 2:15) Apparently, Adam had informed future generations of what God had found acceptable as a sacrifice in the worship of Jehovah, just as he had informed Abel. (Gen 4:4) As Noah and his family came out of the ark on Mount Ararat, Noah “built an altar to Jehovah, and took of every clean beast, and of every clean bird, and offered burnt-offerings on the altar.” (Gen 8:2) Thus, it is obvious that Noah knew was pleasing and acceptable as a sacrifice in the worship of his God.