Career by numbers

Question: A child asked his grandfather: "When you were young, what were you?" The grandfather replied, "I am one hundred, nothing, one thousand, one and one hundred". What's his career?

Question: During what month do people sleep the least?

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: When is 99 more than 100?

Question: What are the next two letters in the following series and why? WATNTLITFS__

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