Riddle #64

Question: The doctor gives you three pills and tells you to take one every half hour. How long before you run out of pills?

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?