Good Riddles

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

Question: What melts in a freezer?

Question: What key is playful?

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

What am I?

Question: Using 3 periods (.) and 1 question mark (?) make this a logical and grammatically correct sentence:

"That that is is that that is not is not is that it it is"