What has many keys but can’t open a single lock?

It's a test that will check your IQ.

Suppose you have one 11 minute hourglass and one 13 minute hourglass.

Can you measure exactly fifteen minutes using them ?

Which digit occurred maximum times between the number 1 and 1000?

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

What does below rebus means ?

A time long back, there lived a king who ruled the great kingdom of Trojan House. As a part of the renovation of the kingdom to meet future security needs, he asked his chief architect to lay down a new play in a manner that all of his 10 castles are connected through five straight walls and each wall must connect four castles together. He also asked the architect that at least one of his castles should be protected with walls. The architect could not come up with any solution that served all of King's choices, but he suggested the best plan that you can see in the picture below. Can you find a better solution to serve the king's demand?

What does the following represent?

N N N N N N N

A A A A A A A

C C C C C C C

John enters a small room. The Door Closes.
When the door opens, John is in a larger room.

Explain?

Two guards were on duty outside a barracks. One faced up the road to watch for anyone approaching from the North. The other looked down the road to see if anyone approached from the South. Suddenly one of them said to the other, "Why are you smiling?"

How did he know his companion was smiling?

Can you name some countries which do not have the alphabet 'A' in them?