Suppose you had 8 billiard balls, and one of them was slightly heavier, but the only way to tell was by putting it on a scale against another. What's the fewest number of times you'd have to use the scale to find the heavier ball?
Put 4 balls each in Pan A & Pan B find the pan having the heaviest ball. Let it be pan b. So all 4 balls in Pan A are good balls, keep it seperate. Now group the balls in pan b into two sets 2 in each pan. Find the Pan having the heaviest ball Let it be Pan A. Now you have identified 6 good balls.
Take one ball from Pan A and weigh it against any one of the 6 good balls if both are equal then the other ball is the culprit else this one should be the culprit.