One of the coins A, B and C, is a fake, heavier than the other two. We only need to use a balance scale once to determine which is the fake.

• What we did:
  • We put A on the scale's left side
  • We put B on the right side
  • The scale balanced
• What does this mean?
  • A and B are exactly the same weight.
  • Obviously, the fake isn't on the scale.
  • Coin C must be the counterfeit.