There are four small balls with no difference in appearance, and their weight may be different. Take a balance, put A and B into one group, and C and D into another group, and place them on both sides of the balance. The balance is basically balanced. Reverse B and D. A and B are obviously heavier than B and C on the other side. It is odd that we put A and C on one side of the balance, and put B on the other side, when we weren't able to put D, the balance was pressed to B side.
heavier, A or D?
 D is heavier than A