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.

Always in you, Sometimes on you;

If I surround you, I can kill you.

What am I?

What jumps when it walks and sits when it stands?

If an electric train is travelling south, then which way is the smoke going?

Find the next number in the series

12, 20, 40, 72, 116, 172 ?

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

Can you find the key in the picture below?

I speak without a mouth, and hear without ears. I have no-body, but I come alive with the wind. What am I?

How many letters are in the alphabet?

There is a square piece of paper with a hole that is denoted by the circle on the top right side in the given picture. You have to cut the paper in a manner that it forms two and only two separate pieces of paper and then rearrange the pieces in a manner that the holes come in the centre of the paper.