A bird resting on the floor of an air-tight box, carried by a boy, starts flying. The boy will feel that the box is now ______.

When a bird flies inside an air-tight box, it pushes air downward with its wings. By Newton's third law, this creates an equal upward force on the bird, and the downward force on the air is transmitted to the box bottom. The total downward force on the box equals the bird's weight, so the weight reading remains unchanged.