On a commercial airliner, there is a hole in one of the layer of the passenger windows.
Or in more detail:
Can you explain the physical reasons for this design choice?
Aircraft windows are in multiple layers thick. The side facing the passengers is just a plastic sheet, since they wouldn't want the passengers scratching up the actual glass window.
However, since pressure changes in the cabin during the flight, it's also necessary to equalize the pressure between the window panes, and hence the little hole.
This picture includes the electric shade system of the 787, but you still get an idea of the different layers of the window.
I'd guess they might have been able to put the hole facing sideways and hidden away internally, but this might perhaps cause problems with condensation, or perhaps risk sucking in dust, but this is just a guess.