This is a puzzle I invented today after noticing something strange. It is possible, however, that you've read it elsewhere in another form. Hope you'll enjoy solving it.
A small town had a public library. The library was manned by only two people: the director of the library and John, the son of a local farmer. John did not have much of an education, but had plenty of good will and was strong -- exactly what you need to move heavy tomes from a shelf to another!
One day the library received a gift: an ancient encyclopedia in forty volumes. The director asked John to get the books out of the cardboard boxes and sort them on the shelves.
At the end of the day, he went to see the job. John had put all the volumes on the shelves as requested, but there was something odd.
Volume 9 was just after volume 4. Volume 19 was immediately after volume 14. Volume 29 was just after volume 24, and volume 39 was just after 34. And volume 40, instead of being the last volume, had been placed roughly in the middle. The rest of the volumes was sorted correctly.
When the director asked John the reason of this, the kid replied: "What do you mean, Sir? They're in order!"
The director was puzzled at first... then it dawned on him.
Why did John put some of the volumes out of place?