Riddle #347

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: What do you call a sad bird?

Question: What kinds of balls do dragons play soccer with?

Question: How are doughnuts and golf alike?

Question: What can be broken but never forgotten?

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?