Good Riddles

Question: All things I devour: Birds, beasts, trees, flowers.
I gnaw iron, bite steel.
I grind hard stones to meal.

What am I?

Question: A man goes down to the beach and starts walking on the beach, he has to get to the circus in a hurry and so he practices his act on the way, even though he had walked for an hour his feet weren't the least bit tired, why?

Question: What Two Things Always Ask A Question But never Answer It?

Question: I plow and plow, but never sow.

What am I?

Question: Tom and Joe want to sit behind each other in class and the teacher arranges them so they are both happy.

How did the teacher do so?