What always comes into a house through the keyhole?

Can you find the hidden animal in the picture below?

I am weightless, but you can see me. Put me in a bucket, and I'll make it lighter. What am I?

A certain street contains 100 buildings. They are numbered from 1 to 100. How many 9's are used in these numbers?

Why do you find that most of the buildings are deprived of the 13th floor?

A network of 20 x 10 squares is given to you.

Can you calculate how many unique squares and rectangles can be formed combining two or more individual squares ?

Different lights make me strange by changing me into different sizes. What am I?

A Scientist was fifty years old in the year 2000 but he was forty years old in the year 2010. How can this be possible?

PS: It has a logical answer.

The first person saw the bridge step on it and crossed,
the second person saw the bridge did not step on it but crossed,
the third person did not see the bridge did not step on it but crossed.
Who are these people?

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?