There are four small balls that do not appear to be separate, but their weights may be different. Take a scales and group A and B into one group, and C and D into another group.
Place them on each side of the scales. The scales is basically balanced. B and D are reversed, then A and B are obviously heavier than B and C. But strangely, we put A and C on one side of the scales, and B on the other side.
When there was no time to put D, the balance was pressed to B's side.
 
Please judge, A or D, which one is heavier?