This one is quite easy and I heard it was one of the Google interview questions. Divide the balls in 3 groups of 3 balls, lets call it Group A/B/C Test A and B, if its the same weigh, the ball must be in the group of the C, test C1 and C2, the heavier one will appear, if not its C3. In case any of A or B weighs more, just find the ball that weighs more of that group like said above.