The square root of number 121 is "11". What is the square root of the number "12345678987654321."?

An American nightclub called 'The Coconut Grove' had a terrible fire in which over 400 people died. A simple design flaw in the building led to a high death toll. Subsequently, regulations were changed to ensure that all public buildings throughout the country eliminated this one detail, which proved deadly. What was it?

Can it be possible to build a house that has all its four walls facing towards the South?

Do You See It?

If eleven plus two equals one, what does nine plus five equal?

How will you measure 15 minutes using two hourglasses of 7 minutes and 11 minutes respectively?

A man started to town with a fox, a goose, and a sack of corn. He came to a stream which he had to cross in a tiny boat. He could only take one across at a time. He could not leave the fox alone with the goose or the goose alone with the corn. How did he get them all safely over the stream?

A rubber ball keeps on bouncing back to 2/3 of the height from which it is dropped. Can you calculate the fraction of its original height that the ball will bounce after it is dropped and it has bounced four times without any hindrance ?

You walk into a room that contains a match, a kerosene lamp, a candle and a fireplace. What would you light first?

It is a six-letter word.
The first four letters are me.
The second and last letters are the same.
The fourth second and last letter is payment.

Who is it?