Which number should replace the question mark?

A Scientist was fifty years old in the year 2000 but he was forty years old in the year 2010. How can this be possible?

PS: It has a logical answer.

How can you write nineteen in a manner that if we take out one, it becomes twenty?

Why Aeroplane windows are round?

The Little ant seems to be always confused. Do you know why?

John is the son of Jacob.
So,
Jacob is the_____of John's father?

A convention is held where all the big logicians are summoned. The master places a band on everyone's forehead. Now all of them can see others bands but can't see his own. Then they are told that there are different colours of bands. All the logicians sit in circle and they are further explained that a bell will ring at regular intervals. The moment when a logician knew the colour of band on his forehead, he will leave at the next bell. If anyone leaves at the wrong bell, he will be disqualified.

The master assures the logicians that the puzzle will not be impossible for anyone of them. How will the logicians manage ?

I am the beginning of the end, and the end of time and space. I am essential to creation, and I surround every place. What am I?

Using mathematical symbols like +, -, and x in order to make the equations correct. Replace # with the above-given symbols.

9 # 8 # 7 # 6 # 5 # 4 = 91

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