This riddle was offered up to students by their teacher. Anyone want to try and guess the answer. If you already know what the answer is don't spoil it for those that may want to guess it.
On a similar note I used to help a friend mark maths test papers for year 5's.
One question was in two parts, the second part being "How did you arrive at your answer?"
One lad put "I guessed"! I thought he should have got a mark for that!
On the same theme, what occurs once a minute, twice in a moment but never in a thousand years