Weight Balance

Divide and conquer in threes.

First weigh three coins against three others. If they balance, the heavier coin is among the three you haven't weighed—then weigh any two of those three and whichever side tips has the heavy coin, or if they balance it is the coin you didn't weigh. If the first weighing tips, take the three coins from the heavier side, weigh two of them; whichever side tips contains the heavy coin, and if they balance it's the third coin from that group.