It walks on 4 when its young, walks on 2 when it's older, and walks on 3 when its very old?

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?

When Jack was six years old he hammered a nail into his favourite tree to mark his height. Ten years later at age sixteen, Jack returned to see how much higher the nail was. If the tree grew by five centimetres each year, how much higher would the nail be?

What am I?

Identify the hidden meaning behind this picture?

Can you replace the question mark with the correct number in the below table sequence?

There was an aeroplane crash, every single person died, but two people survived. How is this possible?

I am a normal creature, but I have five hands. I can even have more but even then I will be considered normal.

Who am I?

Which tank will fill first?

There was a strange boy who was born before his father. How can this be possible?