Riddle #1173

Question: If it takes three men two days to dig a hole, how long does it take one man to dig half of a hole?

Question: What do you call a sad bird?

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

Question: What can be broken but never forgotten?

Question: Where do all the letters sleep?

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?