Good Riddles

Question: 1 is 3, 3 is 5, 5 is 4, 4 is universal. 2 is 3, 3 is 5, 5 is 4, 4 is universal... how is this possible? (What is the rule to this sequence of numbers?)

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

What am I?

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?