Answer:
I know a word of letters three. Add two and fewer there will be. What is the word?
I am a tale in childrens' minds. Though an illusion, I occur every night. I give them a fantasy, I give them a fright. What am I?
What can travel around the world while staying in a corner?
I always have to work with something in my eye.
I shift around though always slowly. I never move more than a few inches at a time. A large movement by me can kill many people. I am huge yet unseen by humans.
No matter how little or how much you use me, you change me every month.