Relate the statement you can't get something for nothing to the law of conservation of mass

The law of conservation of mass states that a mass of an object or the amount of matter composed inside an object or thing will not change unless it is added or removed of the matter that is contained in the object itself.

This means that for example, when you're bag has a mass of a 1kg, and you filled it with clothes it will jump its mass from 1kg to possibly 5kg, hypothetically. Thus, withoout the object or clothes that was added the weight of the bad wouldn't change and vice-versa.