First, if you are up at the Edinburgh Festival, I am speaking about strange stuff on Saturday. Details here.
Please do NOT post your answer, but do say if you think you have solved the puzzle and how long it took. Solution on Monday.
What’s the fewest number of marks you could put on a 12-inch ruler and still be able to measure every distance from 1 inch to 12 inches?
UPDATE: By ‘every distance’, I mean every inch (1 inch, 2 inches, 3 inches etc.).
I have produced an ebook containing 101 of the previous Friday Puzzles! It is called PUZZLED and is available for the Kindle (UK here and USA here) and on the iBookstore (UK here in the USA here). You can try 101 of the puzzles for free here.