I came across this question while looking around the forums at physlink.com: There are birds of mass 20 tonnes in a van of mass 10 tonnes. The back of the van is closed(no air can go in or come out). If now the birds start flying, will the weight of the van decrease? I ask this question here because the folks at physlink at quite divided on this topic. My opinion is that the weight will not decrease since the bird will apply a force on the air to fly. The air will apply the same force on the floor of the van(since it can't go out). Hence the weight shouldn't decrease. Another analogy is that the birds are acting as point particles. They will interact with other birds and with air particles(like another gas mixed in air). So the weight won't decrease. Another question: Can a bird in a closed container lift the container up? This is surely possible if the bird is in a cage, but probably not in this case.