There are 13 balls in same shape and color, numbered by 1, 2, ... , 13 respectively:
12 balls out of them are same in weight, except one, which is either heavier or lighter. Now please use a balance, at most 3 times, to tell which ball is bad.
If you feel it is a quite difficult problem, now please let me help you. Please do steps shown in below:
  1. Select a ball number (1-13) in your brain, make sure it is either heavier or lighter in weight
  2. Click
  3. You will see 4 balls in each side of the balance shown in below.
  4. Try to find whether or not the ball you selected is in the left or right side.
  5. If it is, as you know it is heavier or lighter, the balance is not balanced and you must know which side of the balance will be heavier, click the button under the heavier side.
  6. If it is not, click the button 'Balanced'
  7. After you click any one of the buttons, I will show you a new balance with 4 new balls in each side. Please go to step 4.


Left side of balance
Right side of balance
Which side is heavier? Click one of the buttons shown below