How many sides does a circle have?

When my father was 31 I was 8. Now he is twice as old as me. How old am I?

I cry without having a voice.
I fly without having wings.
I bite without having teeth.
I whisper without a mouth.

Who am I?

Why do you find that most of the buildings are deprived of the 13th floor?

What is full of holes but still holds water?

I am lying and I always lie. Tell me whether I am lying or telling the truth.

What you cannot hold but it is yours?

I drive at an average speed of 30 miles per hour to the railroad station each morning and catch my train. On a particular morning, there was a lot of traffic and at the halfway point, I had averaged only 15 miles per hour. How fast must I drive for the rest of the way to catch my train?

Once upon a time, three rivals got together in a dark wood on an open spot to compose a quarrel by using guns. It was a kind of duel but there were three people in it and therefore it was a truel. There were A, B and C. There were some rules to be followed in the game. The rules of the games were:
-They have to draw the lots in order to find who will fire first who will be second and who will be third.
- The next rule is that they will start firing on each other unless there is a last person alive.
- Each person will decide on his own about which person he will fire on.
-it is quite clear that A hits and kills in all the shots and it is 100%. Whereas 80% of the shots are hit by B and C hits 50% of the shots he tries to fire. the three persons have their strategies for firing. Nobody is killed by the stray bullet.

Question: who will have the highest chance of surviving in the truel and how much is the chance?

What is the riddle which can be asked all day with a different correct answer each time.