PLZ SOLVE THIS

Three wisemen bring 9 pieces of gold (3 pieces from each wiseman) to offer Jesus. Wanting to know that Joseph (Jesus' father) is also wise they give him a riddle. They say:

"Of these 9 pieces of gold, 8 are identical and 1 is slightly heavier. Here is a balance (a two-sided pair of scales). You may load the scales two times to determine which coin is the slightly heavier coin."

Joseph was a wise man also and succeeded in determining which coin was slightly heavier by loading the scales just two times. How did he do it?