What occurs twice in a week, once in a year but never in a day?

What is a word made up of 4 letters, yet is also made up of 3. Sometimes is written with 9 letters, and then with 4. Rarely consists of 6, and never is written with 5.

A boy and a girl are sitting on the porch.
"I'm a boy," says the child with black hair.
"I'm a girl," says the child with red hair.
If at least one of them is lying, who is which?

How could a baby fall out of a twenty-story building onto the ground and live?

John named his first son Alpha, second Beta, third Charlie and fourth Delta. What is the fifth son's name?

A mother is 21 years older than her child. In exactly 6 years from now, the mother will be exactly 5 times as old as the child.
Where's the father?

In a jungle where there are no street lights or any other artificial source of lights, I notice a black snake crossing the road.
How did I get sight of the snake?

There was a blind man. He had four socks in his drawer either black or white. He opened it and took out two socks. Now the probability that it was a pair of white socks is 1/2.

Can you find out the probability that he had taken out a pair of black socks ?

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

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