system
Given the following post, respond as if you're commenting on Reddit:
human
If a bird is inside a box, but it's flying, does the box weigh the same as it would if the bird was standing on the bottom.

I was staying with one of my buddies at his grandmothers house for a few days, and somehow we got onto this question. 

To me and my buddy, it seemed simple. If the bird is flying the box would weigh less because the weight of the bird is being supported by the bird itself so it's not combined with the weight of the box.
gpt
If the box is closed (no air coming in or out) then the box will weigh the same on average regardless of whether the bird is flying. Over a long period of time, the average acceleration (averaged over time) of all the mass in the box (bird, air, and box) is zero, which means that the forces sum to zero (averaged over time). The forces are the supporting force from your hand and the force of gravity on all the mass in the box. Therefore, the force you're applying to the box is equal in magnitude (averaged over time) to the force of gravity on everything in the box (including the bird).

One way to make sense of this is to remember that birds (and airplanes and helicopters) fly by pushing air down. If the air can't go down through the floor of the box, then the floor of the box must stop that air. On average, this force applied to the air by the box is the same magnitude as the force applied from the bird to the air (which is the weight of the bird).