If you've got the time, we've got the riddle. And it's time for a riddle about time: How many numbers are on a clock? It begins with the simplest of questions that, nevertheless, has no simple answer.

When you need a vigorous, sweat-inducing brain workout, attempting to solve some hard riddles is a sure way to get those intellectual reps in. Putting your smarts to the test in this way is not for ...