I read the following problem in a book "A bunch of flies are in a capped jar. you place the jar on a scale. The scale will register the most weight when the flies are a) sitting on the bottom of the jar b) flying around inside the jar c) ... weight of the jar is the same in both cases" The answer from the book about this problem is c). But I don't understand if the flies are sitting on the bottom, I think we should add the weights of flies even it is so tiny (if we consider the scale is sensitive enough for no matter how small the weight is?). But why the answer will be c)?