First take any 4 boxes, divide them into two groups and weigh them. (1) if the weight is the same, take any one of the remaining two boxes instead of any one of the original two groups. What has changed is this box, and what has not changed is the remaining box. (2) If it is not the same weight (it needs to be weighed three times), replace one group with the remaining two boxes. If the balance is balanced, the same as ①; If the balance is unbalanced, it means that the heavier or lighter box is in the original group of the balance, and then one box in their group is replaced by a box under the balance. If the balance is balanced, the removed box will be heavier or lighter. If the balance is unbalanced, the box left on the balance will be heavier or lighter.