Which number should replace the question mark?

Can you convert the number zero(0) into a positive number using just one mathematical operator ?

John said, "If yesterday was tomorrow, today would be Friday."

When did John make this statement?

In 1990, a person is 15 years old. In 1995, that same person is 10 years old. How can this be?

Can you name any English verb that becomes its past tense by simply rearranging the alphabets ?

A Car driver was heading down a street in Washington. He went right past a stop sign without stopping, he turned left where there was a 'no left turn' sign and he went the wrong way on a one-way street. Then he went on the right side of the road past a cop car. Still, he didn't break any traffic laws. Why not?

You need to remove four matchsticks from the picture below and still need to maintain the following as in the original image.
1. 1st row contains 12 matchsticks
2. 2nd row contains 12 matchsticks
3. 1st column contains 12 matchsticks
4. 2nd column contains 12 matchsticks

In the picture, you can see a chess board. On the top left position, the K marks a knight. Now, can you move the knight in a manner that after 63 moves, the knight has been placed at all the squares exactly once excluding the starting square?

What can you catch but never throw?

Can you count the number of seconds in a year?

Clue: You need to bother about calculation.