Every Letter Has An Opposite

K: "Poppa, what's the opposite of 1 million?"
Me: "What do you think it is?"
K: (thinks for 10 seconds or so) "Negative 1 million?"
Me: "Right.  What does opposite mean?"
K: "You just flip it around....what's the opposite of 'c'?"
Me: (confused, but somewhat excited that we may be talking algebra!) "Hmmm...what do you think?"
K: "x"
Me: "How is that opposite?"
K: "a, b, c, so z, y, x"
Me: "I never really though about opposites of letters."
K: "Every letter has an opposite."
Me: "Does every number have an opposite?"
K: "Yes...(long pause) except there's one number that doesn't."
Me: (intrigued) "What number?"
K: "Zero...unless you count zero."

After I grabbed some paper to write down this deliriously wonderful conversation, I asked some follow up questions:

Me: "What's the opposite of x?"
K: "c...is that right?"
Me: "I guess that depends.  What would you say is the opposite of a?"
K: "z"
Me: "What would you say is the opposite of z?"
K: "a"
Me: "So what is the opposite of x?"
K: "Oh, it has to be c. Opposites come in pairs"

I love these conversations!  I never would have thought that the opposite of c was x!