Logic Riddles

Question: Mr. Smith has 4 daughters. Each of his daughters has a brother.

How many children does Mr. Smith have?

Question: There are two sisters: one gives birth to the other and she, in turn, gives birth to the first. Who are the two sisters?

Question: What is the only question you can't answer yes to?

Question: There is a 5 gallon and a 3 gallon box. You also have a hose with unlimited water. How are you gonna make the 5 gallon have 4 gallons using your items? (You do not know the exact measure measurements of a gallon)

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.