How many sides does a circle have?

When I got four, I have it none.
When I got two, I have it some.
When I got none, I have it all.

What is it?

Eight of us go fourth, not back, to protect our king from an attack. What are we?

When we see 2 but call 10?

Find three numbers such that

* When we multiply three numbers, we will get the prime numbers.
* The difference between the second and the first number is equal to the third and second.

Can you name three consecutive days without using the words Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, or Sunday?

In the picture, you can see a chess board. On the top left position, the K marks a knight. Now, can you move the knight in a manner that after 63 moves, the knight has been placed at all the squares exactly once excluding the starting square?

Can you find the six hidden animals in the picture below?

The first box has two white balls. The second box has two black balls. The third box has a white and a black ball.

Boxes are labeled but all labels are wrong!

You are allowed to open one box, pick one ball at random, see its colour and put it back into the box, without seeing the colour of the other ball.

How many such operations are necessary to correctly label the boxes?

Can you think of any three-dimensional shape that comprises of exactly two surfaces?