When I get multiplied by any number, the sum of the figures in the product is always me. What am I?

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

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

He digs out tiny caves and stores gold and silver in them. He also built bridges of silver and made crowns of gold. They are the smallest you could imagine. Sooner or later everybody needs his help, yet many people are afraid to let him help them. Who is he?

Before Mount Everest was discovered, what was the highest mountain in the world?

Do You See It?

What you cannot hold but it is yours?

What word begins and ends with an E, but has only one letter?

I came first on earth but second on the heaven.
I also came twice in a week but found just once in a year.
I stay away from months but you can find me in February.

What am I?

What is the Surname of Barbie Doll?