A two-pan scale seems like a simple tool. But when the uncertainty is double—you don't know which ball is different or whether it weighs more or less—a single weighing may not be enough.
The scale and the different ball
Riddle statement
You have a balance with two pans and three visually identical balls.
You know that one of them weighs differently than the other two, but you don't know if it is heavier or lighter.
Can the different ball be identified with certainty using only one weighing?