Mom, you can never go back to zero. You have to always get bigger, not smaller.

That's right, you're always growing. 

When you're 80, you can't go back to 18. You only go up.

That's true.

Well then I think you should have as much fun as you want each day, since you can't do that day again.

You're right.

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The secret of life, as recognized by a human not even five years on this planet. Well, five that I am aware of, anyhow. My son often has insights like this - like he's closer to 95 than 5. It reminds me to stop & listen to my children when they talk. Often they're doing more than just talking.