Good answer: Weigh 3 coins against 3 coins. If one of the groups is heavier, weigh one of its coins against another one of its coins; this allows you to identify the heavy coin. If the two groups balance, weigh the two leftover coins.

Not‐so‐good answer: Weigh 4 coins against 4 coins. Discard the lighter coins, and weigh 2 coins against 2 coins. Discard the lighter coins and weigh the remaining two coins.