The following puzzle is an old classic, due originally to the great puzzler Sam Loyd, who published the first three parts in 1859; the fourth and fifth parts are variations due to Brian Stewart and Friedrich Amelung respectively. I searched for this puzzle on PSE and couldn't find it, but such a neat chess problem really deserves a place here, so here goes!
The fact that this is an old chestnut means, of course, that you could easily find an answer on the internet in just a few seconds of Googling. But please don't! This will be much more fun to solve if you do it using your wits rather than a search engine :-) Of course I won't be able to tell whether you did or not, but you will always know ...
King Charles XII of Sweden was playing chess with one of his ministers during the Skirmish at Bender. The game reached the following position, with the king playing white and it being his turn to move.
At this point in the game, the king announced a mate in three.
Before he could make his move, a Turkish bullet came whizzing through the window and shattered the white knight. Unperturbed, the king announced that he could still win even from the new position, but this time with a mate in four.
In an incredible stroke of bad luck, another Turkish bullet shattered the pawn at h2 before the king could make his move. This time, he announced a mate in five.
The minister cleverly pointed out that if the second bullet had taken the other white pawn instead, Charles would still have had a mate in ten.
Finally Charles responded by pointing out that if the first bullet had taken the rook instead of the knight, he would at that point have had a mate in six.