What Am I Riddles

Question: In a marble hall as white as milk, Lined with skin as soft as silk, Within a fountain crystal clear, A golden orb does appear, There are no doors to this stronghold, Yet thieves break in and steal the gold. What am I?

Question: Travel a mile and I will change.
Travel a million and I will end as I started.

What am I?

Question: I can be seen, I can be not. Thanks to me, you'll fall in love. I mostly am sweet, but once I were sour. You can receive me, you can't just buy me. You only can send me, or simply share me. What Could I be?

Question: I am almost nonexistent through sorrow, yet am frequent through happiness. I have no contact, yet I am contagious. What am I?

Question: You put me on a table and cut me, but you don't eat me.

What am I?