Good Riddles

Question: What has classes but moves?

Question: How can you add 9 to become 6? (No subtracting).

Question: I can run but never walk. Wherever I go, thought are close behind me.

What am I?

Question: Given to all man then taken away, you can beg for more time but I never stay What am I?

Question: While you were folding a letter I got you.
Usually you will do whatever it takes to avoid me,
but now you can't help but find me.

What am I?