Noah: which animals to take in

Parshat No'ah

In 7:2-3 God tells Noah about taking 7 pairs of pure animals into the ark and 2 of impure animals.

My question is how would Noah know which animals were considered pure and which were considered impure - those rules were only given much later. At that point they would have had no idea, as no animal could be eaten - even the pure ones. So could Noah have known what to take, and what ramification would such a distinction have had?

Anybody?