the bag

Question: You have bag, you put a lump of metal into the bag, then you put a lama into the bag, then you put something else into the bag, and yet the bag weighs nothing. Why?

Question: People see through me, but none pass through without killing me. I can be anywhere, and can be seen double or single. What am I?

Question: During what month do people sleep the least?

Question: When is 99 more than 100?

Question: What can be broken but never forgotten?

Question: The more you add, the less it weighs.