Find a 9-digit number, which you will gradually round off starting with units, then tenth, hundred etc., until you get to the last numeral, which you do not round off. The rounding alternates (up, down, up ...). After rounding off 8 times, the final number is 500000000. The original number is commensurable by 6 and 7, all the numbers from 1 to 9 are used, and after rounding four times the sum of the not-rounded numerals equals 24.

Sherlock Holmes was challenged to make a pre-drawn line smaller without erasing it. He did it in fractions of seconds. How?

A prisoner is told: "If you tell a lie, we will hang you and if you tell the truth, we will shoot you." What did the prisoner say to save himself?

What country can make you shiver?

Double it and multiply it by 4. Then divide it by 8 and you’ll have it once more. What number is it?

What two things can you never eat for breakfast?

How can you leave a room with two legs and return with six legs?

I have eight fingers and two thumbs but I do not live. What Am I?

The more you take me, the more you leave me behind.
Who Am I?

Complete the series by replacing "?" with the correct number.

ST ND RD TH '?'