What is both possible and impossible at the same time?

If it took eight men ten hours to build a wall, how long would it take four men to build it?

Two guards were guarding the camp.Guard-1 was looking towards the south to make sure no threat is coming from the road.Guard-2 was looking at the north to make sure no threat is coming from the top. Suddenly Guard-1 ask the Guard-2 why he is smiling?How Guard-1 knows that Guard-2 is smiling?

Can you think of any three-dimensional shape that comprises of exactly two surfaces?

What's the only room from which no one can enter or leave

There is a brick of gold and a brick of iron in a boat (both 10-inch blocks), if they are both dropped into the water which will make the water level higher?

Can you place six X (crosses) in below board without making three in a row in any way ?

Christina, Allison and Lena are 3 daughters of John a well-known Mathematician, When I asked John the age of their daughters. He replied "The current age of her daughters is prime. Also, the difference between their ages is also prime."

How old are the daughters?

By moving exactly three matchsticks can you make the below equation true.It can be solved by 3 ways.

In a jungle where there are no street lights or any other artificial source of lights, I notice a black snake crossing the road.
How did I get sight of the snake?