Logic Riddles

Question: One female dog goes in and there are no other dogs in there so why did dogs come out?

Question: When is a door not a door?

Question: There are 2 fathers and 2 sons going on a fishing trip. Every person catches one fish. In total, there were only 3 fish. How is that so?

Question: A young man walks through the forest. He comes to a bridge. In front of the bridge is a large man carrying an axe. The man says, "If you want to cross this bridge, you must tell me a statement. If I think the statement is true, you will be strangled to death. If I say the statement is false, your head will be chopped off." A few minutes later, the young man walked over the bridge, while the larger man stood pondering. What was the statement the young man had given?

Question: You have a glass of water that looks about half full. How can you tell, only using the glass of water itself, if the glass is half full or not?

The glass is a right cylinder.