Play · Warm-up
Nine balls, one heavy
Nine balls, one slightly heavier. You get the scale twice. Tap a ball to put it on the left pan, tap again for the right pan, and once more to take it off.
The puzzle
How to think about it
Count the outcomes
A balance has three outcomes, not two: left down, right down, or level. Level still tells you something: the ball isn't on the scale. So split the balls into three groups of three and weigh two of the groups. Whatever happens, three suspects are left, and one more weighing settles it.