What Am I Riddles

Question: I am a digit, but two of the ten.
Two of me make a promise.

What am I?

Question: I have all of your knowledge but could fit in your hands.

What am I?

Question: Weight in my stomach,trees on my back,nails in my ribs,it's feet I lack.What am I?

Question: I'm heavy but not backwards.

What am I?

Question: What's always coming, but never arrives?