What is the centre of gravity?

What time is it when an elephant sits on a fence?

If it took eight men ten hours to build a wall, how long would it take four men to build it?

There was once a college that offered a class on probability applied to the real world. The class was relatively easy, but there was a catch. There were no homework assignments or tests, but there was a final exam that would have only one question on it. When everyone received the test paper it was a blank sheet of paper with a solitary question on it: 'What is the risk?'.Most students were able to pass, but only one student received 100% for the class! Even stranger was that he only wrote down one word!
What did he write?

When can we add 2 to 11 and get 1 as the correct answer?

What is the end of everything?

Why do Chinese men eat more rice than Japanese men do ?

John killed hundreds of people mercilessly and yet the police were not able to put her behind the bars. Why?

A man was driving a black truck. His lights were not on. The moon was not out. A lady was crossing the street. How did the man see her?

A word I know,
Six letters contain,
Subtract just one,
And twelve is what remains.