Actually, happy7, the legend says the clean animals were brought in by sevens. It might be 7 pairs or it may be 7 animals such as one bull and six cows. This was long before the laws concerning clean and unclean were defined so how did Noah know?. God told him, maybe. No need to tell anyone else, they were all going to be dead.
That is the thing about legends: the nuisances are never clear.
You can bet there were a boat load of rats on that ship –well more than a pair. Rats are well noted for sneaking on to ships. The cats wouldn’t be far behind. Where there are cats, the dogs will be near – not to mention fleas for all. Good thing about the cats and dogs, most of us really like them.
Here’s a math-science question for you.
How many cubic kilometers of water are need to cover the earth to the height of mount Everest?
Part 2. How many cubic kilometers of water per second are required to do this in 40 days and nights?
Part 3. Is there enough water currently on earth to do this?