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

You balance a broom horizontally on your finger such that your finger is present at the centre of the gravity of the broom (precisely). You mark the spot and cut the broom into two halves. You are left with two pieces - one long piece with the handle and one short piece with bristles and a portion of the handle.

If you weigh both the pieces now, what will happen?
a) The longer piece weighs more.
b) The shorter piece weighs more.
c) Both pieces weigh equally.

Count the number of times the letter "F" appears in the following paragraph:

FAY FRIED FIFTY POUNDS OF
SALTED FISH AND THREE POUNDS
OF DRY FENNEL FOR DINNER FOR
FORTY MEMBERS OF HER FATHER'S FAMILY.

Can you find any error in the following sentence?

A taxi was standing outside a fort at night. A man came out of the fort, opened the gate and hit the taxi driver killing him instantly. He took all the money out of the meter and started running till he was caught by a local sheriff.

In front of you, there are 9 coins. They all look absolutely identical, but one of the coins is fake. However, you know that the fake coin is lighter than the rest, and in front of you is a balance scale. What is the least number of weightings you can use to find the counterfeit coin?

Count the number of balls in picture below:

How can you leave a room with two legs and return with six legs?

Two-person want to cross a river. Only one boat is present on the riverside that can carry only one person at a time.

However, they still manage to cross the river. How is it possible?

Nine marbles are arranged in the image below. Can you slide two marbles to form a square?