What am I?

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: Surrounded by life , yet life is its biggest enemy. But without life there is no need for it to exist. What is it?

Question: During what month do people sleep the least?

Question: A horse jumps over a castle killing a woman, who is she?

Question: When is 99 more than 100?

Question: What makes a hotel hot even when below zero temperatures?