If you’ve got me, you want to share me; if you share me, you haven’t kept me. What am I?

How old is your son? asked a man to his neighbour. My son is five times as old as my daughter and my wife is five times as old as my son. I am twice as old as my wife whereas my grandmother, who is celebrating her eighty-first birthday is as old as all of us put together.

How old is the man's son ?

It can't be seen, can't be felt, can't be heard, and can't be smelt.
It lies behind stars and under hills, And empty holes it fills.
It comes first and follows after, Ends life, and kills laughter.
What is it?

If 5 cats catch 5 mice in 5 minutes, how long will it take one cat to catch a mouse?

What has 13 hearts but no other organs?

You need to climb ten stairs. At every stair, you can either take one step up or you can jump two steps up.

In how many different ways you can climb 10 stairs?

What is the fewest number of coins that would be required in order to make sure each and every coin touches exactly three other coins?

Which country gets ill every time?

I am a type of vehicle that has two wheels and is powered by humans. What am I?
Hint 1: You can ride me for exercise.
Hint 2: I am environment-friendly.

Can you count the number of quadrilaterals in the picture below?