Good Riddles

Question: I have two rings. I move by request. If the mixture isn't right I just sit and rest.

What am I?

Question: How far can a dog run into the woods?

Question: what are the next two letters in this sequence: A, E, F, H, I, K, L, M,

Question: What is as big as you are and yet does not weigh anything?

Question: What do you call 2 Us?