I am white when I am dirty, and black when I am clean. What am I?

Why do people go to bed?

This was asked in an interview with Coa-Cola.

The interviewer has given me 100 marbles(50 white and 50 black) and two empty boxes.
He then told me that he will leave the room and i need to place all the marbles in two boxes.

And When he come back, he will draw a marble from any of the two box and if the marble is white I will be hired.

Also
* No box can be empty.
* All 100 marbles must be placed in one of the two boxes.

So what should I have done?

A father's child, a mother's child, yet no one's son.

Who am I?

Using mathematical symbols like +, -, and x in order to make the equations correct. Replace # with the above-given symbols.

9 # 8 # 7 # 6 # 5 # 4 = 91

By removing one matchstick and adding one matchstick, can you convert the word "ZOO" to an animal name?

What is the next number in the series?

15 29 56 108 208 ?

What is wrong in the below Picture?

How many triangles are there on the puzzle below?

You measure my life in hours
I serve you by expiring
I vanish faster when I am thin
and slower when I am thick
The wind is my energy
What am I?