Answer:
What can't you see, hear or feel, until its too late. What shadows love, and shopkeepers hate?
It's been around for millions of years, but is never more than a month old. What is it?
What does everyone know how to open but not how to close?
I move without wings, Between silken string, I leave as you find, My substance behind.
No matter how little or how much you use me, you change me every month.
I never sleep yet sometimes I weep. I can get angry or just be happy. Wherever you go, I will follow. No matter what you do, I will be there. I cannot die. What am I?