Good Riddles

Question: When you do not know what I am, then I am something. But when you know what I am, then I am nothing. What am I?

Question: I am a digit, but two of the ten.
Two of me make a promise.

What am I?

Question: I have all of your knowledge but could fit in your hands.

What am I?

Question: You have two buckets: one with only white marbles and one with only black marbles. The two buckets have the same number of marbles.

How can you rearrange the marbles to maximize your chances of grabbing a white marble from each?

Question: You have a cube made of 10 x 10 x 10 smaller cubes, for a total of 1000 smaller cubes. If you take off 1 layer of cubes, how many remain?