Two. Divide the eggs into three groups of three each. Place one group on each side of the scale. If they balance, select two eggs from the remaining group and place one on each side of the scale. If they balance, it is the remaining egg; if not it is the heavier egg. If the eggs from the original weighing do not balance, select two eggs from the heavy side and place one on each side of the scale. If they balance, it is the remaining egg; if not it is the heavier egg.