system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why airplane windows have a small hole in it?
gpt
Airplane windows are composed of multiple layers, and the small hole allows the air pressure to equalize between some of the inner layers. It also prevents moisture buildup that could result in condensation that fogs up the view.

Air pressure within airliners varies quite a bit, they are pressurized above ambient pressure during flight but below ground level pressure. Without a hole the layers of the windows would have significant pressure one way or another, imposing undue material stresses.