What you cannot hold but it is yours?

What does this rebus puzzle means ?

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

A boy purchased a book from a bookkeeper and gave him $100.
The cost of the book is $50 but the bookkeeper has no change, so he gets the change from the next shop and returns the boy his $50.

After some time the next shopkeeper came with the $100 note and told the bookkeeper that the note was a fraud, so he took the money back.

How much loss did the bookkeeper face?

What has a bottom at the top?

What number should come in place of the question mark below?

How many triangles are there in the picture below?

I am a seven-letter word but if you remove four of my letters I will have one left. What word am I?

I left my campsite and hiked south for 3 miles. Then I turned east and hiked for 3 miles. I then turned north and hiked for 3 miles, at which time I came upon a bear inside my tent eating my food! What colour was the bear?

What is the end of everything?