What Am I Riddles

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

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

What am I?

Question: When you do not know what I am, then I am something. But when you know what I am, then I am nothing. What am I?

Question: Round like a apple, deep like a cup, but all the kings horses can't pull it up.

What is it?

Question: With one you can't count past six,
nor can the gap between v and x be fixed.
You can read all of these lines,
but can't say what solves me.

What am I?