system
Given the following post, respond as if you're commenting on Reddit:
human
Take a container.Fill it with birds.Weigh the container.If all the birds took flight within the container, it would still weigh the same.How? 

I just saw this on QI, and even though I think it makes sense I can't really figure out why.

*edit 
Asked and answered comprehensively in under ten minutes. Thanks! I was thinking the birds flying was analogous to someone jumping up, which it clearly isn't.
gpt
>Take a container.Fill it with birds.Weigh the container.If all the birds took flight within the container, it would still weigh the same.

Yes\*.

>How? 

For the birds to stay aloft, they must exert a downward force (via their wings pushing the air) equal to their weight.  The air presses down on the box with the same force as the birds' weight (assuming that the box is air tight) , and thus the box weighs the same.  


\**The weight of the box will, in reality, fluctuate very slightly around the target weight as the birds accelerate upward on a wing beat, and then fall downward.  But then again, that's the same effect you'd see if they were all walking around instead of sitting still.*