Question
I’ve always understood that a lot of cheese is not kosher because it contains animal rennet, which constitutes a mixture of milk and meat. Kosher cheese substitutes enzymes. But in recent years, I have found a lot of certified kosher cheese on the market that lists “rennet” as an ingredient. In fact, some of it even says “animal rennet.” This is all while these products are certified by some of the most recognized hechsherim. How is it possible that cheese containing animal rennet is really kosher?

Question
On Rosh Hashana we eat apple and honey as a sign for a sweet year. Apropros, I have a question that no one I know can answer: Why is honey Kosher? I thought that the product of non-Kosher animals was not Kosher as well. Bees are not Kosher, so why is honey Kosher? I appreciate your time and hope you know the answer.  
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Question
I am interested in keeping kashrus. If I kasher my oven, what happens next? If I cook something that is fleishik what happens next? Can I cook milchik in the oven after cooing fleishik? Or vice versa?