One of twelve tennis balls is a bit lighter or heavier (you do not know which) than the others. How would you identify this odd ball if you could use an old two-pan balance scale only 3 times?
You can only balance one set of balls against another, so no reference weights and no weight measurements.

Answer: I would first take all 12, divide by half, put on scale
See which side lighter, then those 6
Divide again to 3-3
Once you get the lighter side
You're left with 3 balls
There are two outcomes but the method is the same
Choose two of the three, measure them
If one is lighter than another, that's your lighter ball
If they're the same, the leftover ball is the lighter ball.