Good Riddles

Question: Had by few but treasured by all,
I'm on the inside and outside and I make men fall.

What am I?

Question: You have two books. One of them is upside-down and the other is rotated so the top of the book is facing you.

What is the sum of the first page in each book?

Question: A girl lives on the 26th floor of a building. She takes the elevator down to the lobby and goes to school each day. When school is over she goes to the 20th floor on the elevator and walks up 6 flights of stairs to her room. But if she is riding with somebody else on the elevator she goes all the way to the 26th floor.

Why does she do so?

Question: An electric train is traveling south. Which way does the smoke go?

Question: A man murders his wife with a knife in their car. Nobody is around to see this. He throws her out of the car being careful not to leave any fingerprints on her body. Next he throws the knife off of a cliff into a gorge where it will never be found and he goes home. An hour later the police call him and tell him that his wife has been murdered and he needs to come to the scene of the crime immediately. As soon as he arrives, he is arrested.

How did they know he did it?