Riddle #360

Question: Which word does not belong in the following list:

Stop cop mop chop prop shop or crop?

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?